fbpx
Wikipedia

Galaxy

April 10, 2024

This article is about the astronomical structure. For Earth's galaxy, see Milky Way. For other uses, see Galaxy (disambiguation).

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity.[1][2] The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars,[3] range in size from dwarfs with less than a thousand stars,[4] to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.

Galaxies are categorized according to their visual morphology as elliptical,[5] spiral, or irregular.[6] Many are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun.[7]

It is estimated that there are between 200 billion[8] (2×1011) to 2 trillion[9] galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly) and is separated from the Andromeda Galaxy (with diameter of about 152,000 ly), its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)

The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter. Most galaxies are gravitationally organized into groups, clusters and superclusters. The Milky Way is part of the Local Group, which it dominates along with the Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids.[10] Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea.[11]

Etymology

The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way, galaxías (kúklos) γαλαξίας (κύκλος)[12][13] 'milky (circle)', named after its appearance as a milky band of light in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so the baby will drink her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way.[14][15]

In the astronomical literature, the capitalized word "Galaxy" is often used to refer to the Milky Way galaxy, to distinguish it from the other galaxies in the observable universe. The English term Milky Way can be traced back to a story by Geoffrey Chaucer c. 1380:

See yonder, lo, the Galaxyë
 Which men clepeth the Milky Wey,
 For hit is whyt.

— Geoffrey Chaucer, The House of Fame[13]

Galaxies were initially discovered telescopically and were known as spiral nebulae. Most 18th- to 19th-century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought of as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies.[16]

Nomenclature

 
Galaxy cluster SDSS J1152+3313. SDSS stands for Sloan Digital Sky Survey, J for Julian epoch, and 1152+3313 for right ascension and declination respectively.

Millions of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC (New General Catalogue), the IC (Index Catalogue), the CGCG (Catalogue of Galaxies and of Clusters of Galaxies), the MCG (Morphological Catalogue of Galaxies), the UGC (Uppsala General Catalogue of Galaxies), and the PGC (Catalogue of Principal Galaxies, also known as LEDA). All the well-known galaxies appear in one or more of these catalogs but each time under a different number. For example, Messier 109 (or "M109") is a spiral galaxy having the number 109 in the catalog of Messier. It also has the designations NGC 3992, UGC 6937, CGCG 269–023, MCG +09-20-044, and PGC 37617 (or LEDA 37617), among others.[17] Millions of fainter galaxies are known by their identifiers in sky surveys such as the Sloan Digital Sky Survey.[18]

Observation history

Milky Way

Main article: Milky Way

Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.[19]Aristotle (384–322 BCE), however, believed the Milky Way was caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions."[20] Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 CE) was critical of this view, arguing that if the Milky Way was sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax, which it did not. In his view, the Milky Way was celestial.[21]

According to Mohani Mohamed, Arabian astronomer Ibn al-Haytham (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,[22] and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere."[23] Persian astronomer al-Biruni (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars."[24] Andalusian astronomer Avempace (d. 1138) proposed that it was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of refraction from sublunary material,[20][25] citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near.[20] In the 14th century, Syrian-born Ibn Qayyim al-Jawziyya proposed the Milky Way galaxy was "a myriad of tiny stars packed together in the sphere of the fixed stars."[26]

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed of a huge number of faint stars.[27][28] In 1750, English astronomer Thomas Wright, in his An Original Theory or New Hypothesis of the Universe, correctly speculated that it might be a rotating body of a huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it.[29][30] In his 1755 treatise, Immanuel Kant elaborated on Wright's idea about the Milky Way's structure.[31]

 
The shape of the Milky Way as estimated from star counts by William Herschel in 1785; the Solar System was assumed to be near the center.

The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center.[32][33] Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center.[30] Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane; but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of the Milky Way galaxy emerged.[34]

Distinction from other nebulae

A few galaxies outside the Milky Way are visible on a dark night to the unaided eye, including the Andromeda Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, and the Triangulum Galaxy. In the 10th century, Persian astronomer Abd al-Rahman al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".[35] In 964, he probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars (referring to "Al Bakr of the southern Arabs",[36] since at a declination of about 70° south it was not visible where he lived); it was not well known to Europeans until Magellan's voyage in the 16th century.[37][36] The Andromeda Galaxy was later independently noted by Simon Marius in 1612.[35] In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what could be seen. These views "are remarkably close to the present-day views of the cosmos."[38] In 1745, Pierre Louis Maupertuis conjectured that some nebula-like objects were collections of stars with unique properties, including a glow exceeding the light its stars produced on their own, and repeated Johannes Hevelius's view that the bright spots were massive and flattened due to their rotation.[39] In 1750, Thomas Wright correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.[30][40]

 
Photograph of the "Great Andromeda Nebula" by Isaac Roberts, 1899, later identified as the Andromeda Galaxy

Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.[30] In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae.[41]

In 1912, Vesto M. Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high Doppler shifts, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us.[42][43]

In 1917, Heber Doust Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier object M31, was then known). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.[44]

In 1920 a debate took place between Harlow Shapley and Heber Curtis (the Great Debate), concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[45]

In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.[46] Using the new 100-inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.[47] In 1926 Hubble produced a classification of galactic morphology that is used to this day.[48][49]

Modern research

 
Rotation curve of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.[50]

In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas;[51] and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of a rotating bar structure in the center of this galaxy.[52] With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.[53][54]

Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars.[55] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe.[56] Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the Zone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.[57]

A 2016 study published in The Astrophysical Journal, led by Christopher Conselice of the University of Nottingham, used 20 years of Hubble images to estimate that the observable universe contained at least two trillion (2×1012) galaxies.[58][59] However, later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion (2×1011).[60][61]

Types and morphology

See also: Galactic bulge, Central massive object, Active galactic nucleus, and Galaxy morphological classification
 
Types of galaxies according to the Hubble classification scheme : an E indicates a type of elliptical galaxy; an S is a spiral; and SB is a barred spiral galaxy

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies.[6]

Many galaxies are thought to contain a supermassive black hole at their center. This includes the Milky Way, whose core region is called the Galactic Center.[62]

Ellipticals

Main article: Elliptical galaxy

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead, they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters.[63]

Type-cD galaxies

 
The galaxy cluster Abell 1413 is dominated by this cD elliptical galaxy designated Abell 1413 BCG. It has an isophotal diameter of over 800,000 light-years across. Note the gravitational lensing.

The largest galaxies are the type-cD galaxies. First described in 1964 by a paper by Thomas A. Matthews and others,[64] they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as the supergiant elliptical galaxies and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales.[65] The profile of their surface brightnesses as a function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts.[66]

The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities.[67] These processes are the mechanisms that drive the formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon of cooling flow, where the heated gases in clusters collapses towards their centers as they cool, forming stars in the process,[68] a phenomenon observed in clusters such as Perseus,[69] and more recently in the Phoenix Cluster.[70]

Shell galaxy

 
NGC 3923 Elliptical Shell Galaxy (Hubble photograph)

A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over 20 shells.[71]

Spirals

Main articles: Spiral galaxy and Barred spiral galaxy
 
The Pinwheel Galaxy, NGC 5457

Spiral galaxies resemble spiraling pinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept.[72]

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) which indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region.[73] A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.[74] The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.[75][76]

 
NGC 1300, an example of a barred spiral galaxy

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves".[77] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.[78]

 
Hoag's Object, an example of a ring galaxy

Barred spiral galaxy

A majority of spiral galaxies, including the Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.[79] In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy.[80] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[81]

Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy[82] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[83] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[84]

Super-luminous spiral

Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 87,400 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than the Milky Way.[85][86]

Other morphologies

  • Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies.
    • A ring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.[87] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.[88]
  • A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars[89] (barred lenticular galaxies receive Hubble classification SB0).
  • Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral morphology.
    • An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.
    • Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.[90] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.[91]
  • A dark or "ultra diffuse" galaxy is an extremely-low-luminosity galaxy. It may be the same size as the Milky Way, but have a visible star count only one percent of the Milky Way's. Multiple mechanisms for producing this type of galaxy have been proposed, and it is possible that different dark galaxies formed by different means.[92] One candidate explanation for the low luminosity is that the galaxy lost its star-forming gas at an early stage, resulting in old stellar populations.[93][94]

Dwarfs

Main article: Dwarf galaxy

Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies.[95] They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.[96]

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.[97] Most of the information we have about dwarf galaxies come from observations of the local group, containing two spiral galaxies (the Milky Way and Andromeda) and many dwarf galaxies. These dwarf galaxies are classified as either irregular or dwarf elliptical/dwarf spheroidal galaxies.[95]

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether it has thousands or millions of stars. This suggests that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.[98]

Variants

Interacting

Main article: Interacting galaxy
 
The Antennae Galaxies are undergoing a collision that will result in their eventual merger.

Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.[99][100] Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but the gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures.[99][100]

At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to the galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known as cannibalism, where the more massive larger galaxy remains relatively undisturbed, and the smaller one is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.[99][100]

Starburst

Main article: Starburst galaxy
 
M82, a starburst galaxy that has ten times the star formation of a "normal" galaxy[101]

Stars are created within galaxies from a reserve of cold gas that forms giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst. If they continue to do so, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during the universe's early history,[102] but still contribute an estimated 15% to total star production.[103]

Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions.[104] These stars produce supernova explosions, creating expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity end.[102]

Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity.[105]

Radio galaxy

Main article: Radio galaxy
 
Hercules A, supergiant elliptical radio galaxy

A radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus.[106] Radio galaxies are classified according to their Fanaroff–Riley classification. The FR I class have lower radio luminosity and exhibit structures which are more elongated; the FR II class are higher radio luminosity. The correlation of radio luminosity and structure suggests that the sources in these two types of galaxies may differ.[107]

Radio galaxies can also be classified as giant radio galaxies (GRGs), whose radio emissions can extend to scales of megaparsecs (3.26 million light-years). Alcyoneus is an FR II class low-excitation radio galaxy which has the largest observed radio emission, with lobed structures spanning 5 megaparsecs (16×106 ly). For comparison, another similarly sized giant radio galaxy is 3C 236, with lobes 15 million light-years across. It should however be noted that radio emissions are not always considered part of the main galaxy itself.[108]

A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole. Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales, reaching upwards to several megaparsecs across, far larger than the diameters of their host galaxies.[109]

A "normal" radio galaxy do not have a source that is a supermassive black hole or monster neutron star; instead the source is synchrotron radiation from relativistic electrons accelerated by supernova. These sources are comparatively short lived, making the radio spectrum from normal radio galaxies an especially good way to study star formation. [110]

Active galaxy

Main article: Active galactic nucleus
 
A jet of particles is being emitted from the core of the elliptical radio galaxy M87.

Some observable galaxies are classified as "active" if they contain an active galactic nucleus (AGN).[111] A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars, dust and interstellar medium. There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are called Seyfert galaxies, while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or quasars. Models of AGNs suggest that a significant fraction of their light is shifted to far-infrared frequencies because optical and UV emission in the nucleus is absorbed and remitted by dust and gas surrounding it.[112]

The standard model for an active galactic nucleus is based on an accretion disc that forms around a supermassive black hole (SMBH) at the galaxy's core region. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.[113][114] The AGN's luminosity depends on the SMBH's mass and the rate at which matter falls onto it. In about 10% of these galaxies, a diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light. The mechanism for producing these jets is not well understood.[115]

Seyfert galaxy

Main article: Seyfert galaxy

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses; but unlike quasars, their host galaxies are clearly detectable.[116] Seen through a telescope, a Seyfert galaxy appears like an ordinary galaxy with a bright star superimposed atop the core. Seyfert galaxies are divided into two principal subtypes based on the frequencies observed in their spectra.[117]

Quasar

Main article: Quasar

Quasars are the most energetic and distant members of active galactic nuclei. Extremely luminous, they were first identified as high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared more similar to stars than to extended sources similar to galaxies. Their luminosity can be 100 times that of the Milky Way.[118] The nearest known quasar, Markarian 231, is about 581 million light-years from Earth,[119] while others have been discovered as far away as UHZ1, roughly 13.2 billion light-years distant.[120][121] Quasars are noteworthy for providing the first demonstration of the phenomenon that gravity can act as a lens for light.[122]

Other AGNs

Blazars are believed to be active galaxies with a relativistic jet pointed in the direction of Earth. A radio galaxy emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the observer's position.[115]

Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements. The excitation sources for the weakly ionized lines include post-AGB stars, AGN, and shocks.[123] Approximately one-third of nearby galaxies are classified as containing LINER nuclei.[114][123][124]

Luminous infrared galaxy

Main article: Luminous infrared galaxy

Luminous infrared galaxies (LIRGs) are galaxies with luminosities—the measurement of electromagnetic power output—above 1011 L☉ (solar luminosities). In most cases, most of their energy comes from large numbers of young stars which heat surrounding dust, which reradiates the energy in the infrared. Luminosity high enough to be a LIRG requires a star formation rate of at least 18 M☉ yr−1. Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr−1. Many LIRGs also emit radiation from an AGN.[125][126] Infrared galaxies emit more energy in the infrared than all other wavelengths combined, with peak emission typically at wavelengths of 60 to 100 microns. LIRGs are believed to be created from the strong interaction and merger of molecular-gas-rich spiral galaxies.[127]: 5.1  While uncommon in the local universe, LIRGs and ULIRGS were more prevalent when the universe was younger.[126]

Physical diameters

Galaxies do not have a definite boundary by their nature, and are characterized by a gradually decreasing stellar density as a function of increasing distance from their center, making measurements of their true extents difficult. Nevertheless, astronomers over the past few decades have made several criteria in defining the sizes of galaxies.

Angular diameter

As early as the time of Edwin Hubble in 1936, there have been attempts to characterize the diameters of galaxies. The earliest efforts were based on the observed angle subtended by the galaxy and its estimated distance, leading to an angular diameter (also called "metric diameter").[128]: 43  This type of measurement is subject to two significant issues 1) the estimated distance to the galaxy must be corrected for the redshift-related space expansion and 2) collections of angular-diameter data are subject to selection bias as more distant observations preferentially select the most luminous objects.[128]: 73 

Isophotal diameter

The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness.[129] Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond (mag/arcsec2; sometimes expressed as mag arcsec−2), which defines the brightness depth of the isophote. To illustrate how this unit works, a typical galaxy has a brightness flux of 18 mag/arcsec2 at its central region. This brightness is equivalent to the light of an 18th magnitude hypothetical point object (like a star) being spread out evenly in a one square arcsecond area of the sky.[130] The isophotal diameter is typically defined as the region enclosing all the light down to 25 mag/arcsec2 in the blue B-band,[131] which is then referred to as the D25 standard.[132]

Examples of isophotal diameters (25.0 B-mag/arcsec2 isophote)
galaxy diameter reference
Large Magellanic Cloud 9.96 kiloparsecs (32,500 light-years) [133]
Milky Way 26.8 kiloparsecs (87,400 light-years) [134]
Messier 87 40.55 kiloparsecs (132,000 light-years) [135]
Andromeda Galaxy 46.58 kiloparsecs (152,000 light-years) [136]

Effective radius (half-light) and its variations

The half-light radius (also known as effective radius; Re) is a measure that is based on the galaxy's overall brightness flux. This is the radius upon which half, or 50%, of the total brightness flux of the galaxy was emitted. This was first proposed by Gérard de Vaucouleurs in 1948.[137] The choice of using 50% was arbitrary, but proved to be useful in further works by R. A. Fish in 1963,[138] where he established a luminosity concentration law that relates the brightnesses of elliptical galaxies and their respective Re, and by J.L. Sérsic in 1968[139] that defined a mass-radius relation in galaxies.[129]

In defining Re, it is necessary that the overall brightness flux galaxy should be captured, with a method employed by Bershady in 2000 suggesting to measure twice the size where the brightness flux of an arbitrarily chosen radius, defined as the local flux, divided by the overall average flux equals to 0.2.[140] Using half-light radius allows a rough estimate of a galaxy's size, but is not particularly helpful in determining its morphology.[141]

Variations of this method exist. In particular, in the ESO-Uppsala Catalogue of Galaxies values of 50%, 70%, and 90% of the total blue light (the light detected through a B-band specific filter) had been used to calculate a galaxy's diameter.[142]

Petrosian magnitude

First described by V. Petrosian in 1976,[143] a modified version of this method has been used by the Sloan Digital Sky Survey (SDSS). This method employs a mathematical model on a galaxy whose radius is determined by the azimuthally (horizontal) averaged profile of its brightness flux. In particular, the SDSS employed the Petrosian magnitude in the R-band (658 nm, in the red part of the visible spectrum) to ensure that the brightness flux of a galaxy would be captured as much as possible while counteracting the effects of background noise. For a galaxy whose brightness profile is exponential, it is expected to capture all of its brightness flux, and 80% for galaxies that follow a profile that follows de Vaucouleurs's law.[144]

Petrosian magnitudes have the advantage that it is redshift and distance independent, allowing the measurement of the galaxy's apparent size since the Petrosian radius is defined in terms of the galaxy's overall luminous flux.[145]

A critique of an earlier version of this method has been issued by IPAC,[146] with the method causing a magnitude of error (upwards to 10%) of the values than using isophotal diameter. The use of Petrosian magnitudes also have the disadvantage of missing most of the light outside the Petrosian aperture, which is defined relative to the galaxy's overall brightness profile, especially for elliptical galaxies, with higher signal-to-noise ratios on higher distances and redshifts.[147] A correction for this method has been issued by Graham et al. in 2005, based on the assumption that galaxies follow Sérsic's law.[145]

Near-infrared method

This method has been used by 2MASS as an adaptation from the previously used methods of isophotal measurement. Since 2MASS operates in the near infrared, which has the advantage of being able to recognize dimmer, cooler, and older stars, it has a different form of approach compared to other methods that normally use B-filter. The detail of the method used by 2MASS has been described thoroughly in a document by Jarrett et al., with the survey measuring several parameters.[148]

The standard aperture ellipse (area of detection) is defined by the infrared isophote at the Ks band (roughly 2.2 μm wavelength) of 20 mag/arcsec2. Gathering the overall luminous flux of the galaxy has been employed by at least four methods: the first being a circular aperture extending 7 arcseconds from the center, an isophote at 20 mag/arcsec2, a "total" aperture defined by the radial light distribution that covers the supposed extent of the galaxy, and the Kron aperture (defined as 2.5 times the first-moment radius, an integration of the flux of the "total" aperture).[148]

Magnetic fields

Galaxies have magnetic fields of their own. A galaxy's magnetic field influences its dynamics in multiple ways, including affecting the formation of spiral arms and transporting angular momentum in gas clouds. The latter effect is particularly important, as it is a necessary factor for the gravitational collapse of those clouds, and thus for star formation.[149]

The typical average equipartition strength for spiral galaxies is about 10 μG (microgauss) or 1 nT (nanotesla). By comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss) or 30 μT (microtesla). Radio-faint galaxies like M 31 and M33, the Milky Way's neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms, the field strength can be up to 25 μG, in regions where cold gas and dust are also concentrated. The strongest total equipartition fields (50–100 μG) were found in starburst galaxies—for example, in M 82 and the Antennae; and in nuclear starburst regions, such as the centers of NGC 1097 and other barred galaxies.[149]

Formation and evolution

Main article: Galaxy formation and evolution

Formation

 
Artist's impression of a protocluster forming in the early universe[150]

Current models of the formation of galaxies in the early universe are based on the ΛCDM model. About 300,000 years after the big bang, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos.[151][152] These primordial structures eventually became the galaxies we see today.[153]

 
Artist's impression of a young galaxy accreting material

Early galaxy formation

Evidence for the appearance of galaxies very early in the Universe's history was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and earliest-to-form galaxy seen at that time.[154] While some scientists have claimed other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in an earlier stage of the universe's evolution), IOK-1's age and composition have been more reliably established. In December 2012, astronomers reported that UDFj-39546284 is the most distant object known and has a redshift value of 11.9. The object, estimated to have existed around 380 million years[155] after the Big Bang (which was about 13.8 billion years ago),[156] is about 13.42 billion light travel distance years away. The existence of galaxies so soon after the Big Bang suggests that protogalaxies must have grown in the so-called "dark ages".[151] As of May 5, 2015, the galaxy EGS-zs8-1 is the most distant and earliest galaxy measured, forming 670 million years after the Big Bang. The light from EGS-zs8-1 has taken 13 billion years to reach Earth, and is now 30 billion light-years away, because of the expansion of the universe during 13 billion years.[157][158][159][160][161] On 17 August 2022, NASA released a large mosaic image of 690 individual frames taken by the Near Infrared Camera (NIRCam) on the James Webb Space Telescope (JWST) of numerous very early galaxies.[162][163][164]

In May 2023, a study in the journal Nature identified an ultra-faint galaxy named JD1.[165] Galaxy JD1 was observed by the JWST using the near-infrared spectrograph instrument NIRSpec and was found to have a distance value of redshift z=9.79.[165] This means that JD1 was observed at 480 million years after the Big Bang when the universe was only about 4% of its present age.[165][166] Observations of this ultra-faint galaxy were aided by the effect of a gravitational lens in the galaxy cluster Abell 2744 which helped make the image of JD1 larger and 13 times brighter than it otherwise would be.[165][166] This effect and the use of the JWST's NIRCam showed JD1's structure to be three starforming clumps of dust and gas.[166]

 
Different components of near-infrared background light detected by the Hubble Space Telescope in deep-sky surveys[167]

The detailed process by which the earliest galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. In top-down correlations (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.[168] In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.[169] Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. These were composed almost entirely of hydrogen and helium and may have been more massive than 100 times the Sun's mass. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium.[170] This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.[171]

In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.[172][173]

In late 2023, scientists reported results suggesting that newborn galaxies in the very early universe were "banana"-shaped, much to the surprise of researchers.[174][175]

Evolution

Within a billion years of a galaxy's formation, key structures begin to appear.[176] Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.[177] During this early epoch, galaxies undergo a major burst of star formation.[178]

During the following two billion years, the accumulated matter settles into a galactic disc.[179] A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life.[180] This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.[181]

Hubble eXtreme Deep Field (XDF)
 
XDF view field compared to the angular size of the Moon. Several thousand galaxies, each consisting of billions of stars, are in this small view.
 
XDF (2012) view: Each light speck is a galaxy, some of which are as old as 13.2 billion years[182] – the observable universe is estimated to contain 200 billion to two trillion galaxies.
 
XDF image shows (from left) fully mature galaxies, nearly mature galaxies (from five to nine billion years ago), and protogalaxies, blazing with young stars (beyond nine billion years).

Star formation rates in galaxies depend upon their local environment. Isolated 'void' galaxies have highest rate per stellar mass, with 'field' galaxies associated with spiral galaxies having lower rates and galaxies in dense cluster having the lowest rates.[183]

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology.[184] Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in NGC 4676[185] or the Antennae Galaxies.[186]

The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two might collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, it has collided and merged with other galaxies in the past.[187] Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the Kraken.[188][189]

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked about ten billion years ago.[190]

Future trends

Main article: Future of an expanding universe

Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.[191] Elliptical galaxies are largely devoid of this gas, and so form few new stars.[192] The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.[193][194]

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest, longest-lived stars in the visible universe, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.[193][195]

Larger-scale structures

Main articles: Observable universe § Large-scale structure, Galaxy filament, and Galaxy groups and clusters
 
Seyfert's Sextet is an example of a compact galaxy group.

Deep-sky surveys show that galaxies are often found in groups and clusters. Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past billion years are relatively scarce. Only about 5% of the galaxies surveyed are truly isolated; however, they may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller satellite galaxies.

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see Hubble's law). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This ongoing merging process (as well as an influx of infalling gas) heats the intergalactic gas in a cluster to very high temperatures of 30–100 megakelvins.[196] About 70–80% of a cluster's mass is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent in the form of galaxies.[197]

Most galaxies are gravitationally bound to a number of other galaxies. These form a fractal-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster; these formations contain the majority of galaxies (as well as most of the baryonic mass) in the universe.[198][199] To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see Virial theorem). If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers.[200]

Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.[201] Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over time, tidally destroys its satellite galaxies and adds their mass to its own.[202]

 
Southern plane of the Milky Way from submillimeter wavelengths[203]

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids.[204] Above this scale, the universe appears to be the same in all directions (isotropic and homogeneous),[205] though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The Hercules–Corona Borealis Great Wall, currently the largest structure in the universe found so far, is 10 billion light-years (three gigaparsecs) in length.[206][207][208]

The Milky Way galaxy is a member of an association named the Local Group, a relatively small group of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two.[209] The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on the Virgo Cluster.[210] In turn, the Virgo Supercluster is a portion of the Laniakea Supercluster.[211]

Multi-wavelength observation

See also: Observational astronomy
 
This ultraviolet image of Andromeda shows blue regions containing young, massive stars.

Advances in astronomy have always been driven by technology. After centuries of success in optical astronomy, recent decades have seen major progress in other regions of the electromagnetic spectrum.[212]

The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail.[213] Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.[214]

The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below this range.)[215] Large radio interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via 21 cm radiation), including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies.[216]

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole.[217] The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.[218]

Gallery

See also

Notes

References

  1. ^ Sparke & Gallagher 2000, p. i
  2. ^ Hupp, E.; Roy, S.; Watzke, M. (August 12, 2006). . NASA. Archived from the original on March 28, 2020. Retrieved April 17, 2007.
  3. ^ Howell, Elizabeth; Harvey, Ailsa (February 11, 2022). "How many stars are in the universe? - Can we estimate the total number of stars?". Space.com. Retrieved July 16, 2022.
  4. ^ Strigari, Louis E.; Bullock, James S.; Kaplinghat, Manoj; Simon, Joshua D.; Geha, Marla; Willman, Beth; Walker, Matthew G. (2008). "A common mass scale for satellite galaxies of the Milky Way". Nature. 454 (7208): 1096–1097. arXiv:0808.3772. doi:10.1038/nature07222. ISSN 0028-0836. PMID 18756252.
  5. ^ Hoover, A. (June 16, 2003). . Hubble News Desk. Archived from the original on July 20, 2011. Retrieved March 4, 2011.
    • Based upon: Graham, A. W.; Guzman, R. (2003). "HST Photometry of Dwarf Elliptical Galaxies in Coma, and an Explanation for the Alleged Structural Dichotomy between Dwarf and Bright Elliptical Galaxies". The Astronomical Journal. 125 (6): 2936–2950. arXiv:astro-ph/0303391. Bibcode:2003AJ....125.2936G. doi:10.1086/374992. S2CID 13284968.
  6. ^ a b Jarrett, T. H. "Near-Infrared Galaxy Morphology Atlas". California Institute of Technology. Archived from the original on August 2, 2012. Retrieved January 9, 2007.
  7. ^ Finley, D.; Aguilar, D. (November 2, 2005). "Astronomers Get Closest Look Yet At Milky Way's Mysterious Core". National Radio Astronomy Observatory. from the original on December 20, 2015. Retrieved August 10, 2006.
  8. ^ "Astronomers were wrong about the number of galaxies in universe". The Jerusalem Post. January 14, 2021. from the original on January 14, 2021. Retrieved January 14, 2021.
  9. ^ Saunders, Toby (July 25, 2023). "How many galaxies are in the Universe? A lot more than you'd think". BBC Science Focus. Retrieved January 9, 2024.
  10. ^ "Galaxy Clusters and Large-Scale Structure". University of Cambridge. Archived from the original on May 24, 2012. Retrieved January 15, 2007.
  11. ^ Gibney, Elizabeth (2014). "Earth's new address: 'Solar System, Milky Way, Laniakea'". Nature. doi:10.1038/nature.2014.15819. S2CID 124323774.
  12. ^ C. T. Onions et al., The Oxford Dictionary of English Etymology, Oxford, 1966, p. 385.
  13. ^ a b Harper, D. "galaxy". Online Etymology Dictionary. Archived from the original on May 27, 2012. Retrieved November 11, 2011.
  14. ^ Waller & Hodge 2003, p. 91
  15. ^ Konečný, Lubomír. (PDF). Academy of Sciences of the Czech Republic. Archived from the original (PDF) on July 20, 2006. Retrieved January 5, 2007.
  16. ^ Rao, J. (September 2, 2005). "Explore the Archer's Realm". Space.com. from the original on October 31, 2010. Retrieved January 3, 2007.
  17. ^ "Results for object MESSIER 109 (M109)". NASA/IPAC Extragalactic Database. Retrieved January 13, 2023.
  18. ^ . SkyServer. Archived from the original on January 12, 2024. Retrieved March 3, 2024.
  19. ^ Plutarch (2006). The Complete Works Volume 3: Essays and Miscellanies. Echo Library. p. 66. ISBN 978-1-4068-3224-2. from the original on March 24, 2021. Retrieved July 25, 2018.
  20. ^ a b c Montada, J. P. (September 28, 2007). "Ibn Bâjja". Stanford Encyclopedia of Philosophy. from the original on March 16, 2020. Retrieved July 11, 2008.
  21. ^ Heidarzadeh 2008, pp. 23–25
  22. ^ Mohamed 2000, pp. 49–50
  23. ^ Bouali, H.-E.; Zghal, M.; Lakhdar, Z. B. (2005). "Popularisation of Optical Phenomena: Establishing the First Ibn Al-Haytham Workshop on Photography" (PDF). The Education and Training in Optics and Photonics Conference. (PDF) from the original on May 24, 2011. Retrieved July 8, 2008.
  24. ^ O'Connor, John J.; Robertson, Edmund F., "Abu Arrayhan Muhammad ibn Ahmad al-Biruni", MacTutor History of Mathematics Archive, University of St Andrews
  25. ^ Heidarzadeh 2008, p. 25, Table 2.1
  26. ^ Livingston, J. W. (1971). "Ibn Qayyim al-Jawziyyah: A Fourteenth Century Defense against Astrological Divination and Alchemical Transmutation". Journal of the American Oriental Society. 91 (1): 96–103 [99]. doi:10.2307/600445. JSTOR 600445.
  27. ^ Galilei, Galileo (1610). Sidereus Nuncius. Venice, (Italy): Thomas Baglioni. pp. 15–16. Retrieved January 10, 2024.
    English translation: Galilei, Galileo (1880). The Sidereal Messenger. Translated by Carlos, Edward Stafford. London, England: Rivingtons. pp. 42–43. Retrieved January 10, 2024.
  28. ^ O'Connor, J. J.; Robertson, E. F. (November 2002). "Galileo Galilei". University of St. Andrews. Archived from the original on May 30, 2012. Retrieved January 8, 2007.
  29. ^ Wright, Thomas (1750). An Original Theory or New Hypothesis of the Universe. London, England: H. Chapelle. pp. 48–. November 20, 2016, at the Wayback Machine "... the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space, without order or design, ... this phænomenon [is] no other than a certain effect arising from the observer's situation, ... To a spectator placed in an indefinite space, ... it [i.e., the Milky Way (Via Lactea)] [is] a vast ring of stars ..."
    On page 73 November 20, 2016, at the Wayback Machine, Wright called the Milky Way the Vortex Magnus (the great whirlpool) and estimated its diameter at 8.64×1012 miles (13.9×1012 km).
  30. ^ a b c d Evans, J. C. (November 24, 1998). "Our Galaxy". George Mason University. Archived from the original on June 30, 2012. Retrieved January 4, 2007.
  31. ^ Kant, Immanuel (November 20, 2016) [1755]. Allgemeine Naturgeschichte und Theorie des Himmels (Universal Natural History and Theory of the Heavens). Königsberg and Leipzig, (Germany): Johann Friederich Petersen. ISBN 978-3-89131-002-1. 2016-11-20 at the Wayback Machine  
    Available in English translation by Ian Johnston at: Vancouver Island University, British Columbia, Canada August 29, 2014, at the Wayback Machine
  32. ^ Herschel, William (1785). "XII. On the construction of the heavens". Giving Some Accounts of the Present Undertakings, Studies, and Labours, of the Ingenious, in Many Considerable Parts of the World. Philosophical Transactions of the Royal Society of London. Vol. 75. London. pp. 213–266. doi:10.1098/rstl.1785.0012. ISSN 0261-0523. S2CID 186213203. from the original on November 20, 2016. Retrieved January 27, 2016. Herschel's diagram of the galaxy appears immediately after the article's last page.
  33. ^ Paul 1993, pp. 16–18
  34. ^ Trimble, V. (1999). "Robert Trumpler and the (Non)transparency of Space". Bulletin of the American Astronomical Society. 31 (31): 1479. Bibcode:1999AAS...195.7409T.
  35. ^ a b Kepple & Sanner 1998, p. 18
  36. ^ a b "The Large Magellanic Cloud, LMC". Observatoire de Paris. March 11, 2004. from the original on June 22, 2017.
  37. ^ "Abd-al-Rahman Al Sufi (December 7, 903 – May 25, 986 A.D.)". Observatoire de Paris. from the original on April 16, 2007. Retrieved April 19, 2007.
  38. ^ Gordon, Kurtiss J. "History of our Understanding of a Spiral Galaxy: Messier 33". Caltech.edu. from the original on January 25, 2021. Retrieved June 11, 2018.
  39. ^ Kant, Immanuel, Universal Natural History and Theory of the Heavens (1755)
  40. ^ See text quoted from Wright's An original theory or new hypothesis of the Universe in Dyson, F. (1979). Disturbing the Universe. Pan Books. p. 245. ISBN 978-0-330-26324-5. from the original on March 24, 2021. Retrieved July 25, 2018.
  41. ^ "William Rosse". parsonstown.info. March 24, 2021. Retrieved January 11, 2024.
  42. ^ Slipher, V. M. (1913). "The radial velocity of the Andromeda Nebula". Lowell Observatory Bulletin. 1 (8): 56–57. Bibcode:1913LowOB...2...56S.
  43. ^ Slipher, V. M. (1915). "Spectrographic Observations of Nebulae". Popular Astronomy. Vol. 23. pp. 21–24. Bibcode:1915PA.....23...21S.
  44. ^ Curtis, H. D. (1988). "Novae in Spiral Nebulae and the Island Universe Theory". Publications of the Astronomical Society of the Pacific. 100: 6. Bibcode:1988PASP..100....6C. doi:10.1086/132128.
  45. ^ Weaver, H. F. "Robert Julius Trumpler". US National Academy of Sciences. from the original on December 24, 2013. Retrieved January 5, 2007.
  46. ^ Öpik, E. (1922). "An estimate of the distance of the Andromeda Nebula". The Astrophysical Journal. 55: 406. Bibcode:1922ApJ....55..406O. doi:10.1086/142680.
  47. ^ Hubble, E. P. (1929). "A spiral nebula as a stellar system, Messier 31". The Astrophysical Journal. 69: 103–158. Bibcode:1929ApJ....69..103H. doi:10.1086/143167.
  48. ^ Hubble, Edwin. (1926). "No. 324. Extra-galactic nebulae". Contributions from the Mount Wilson Observatory. 324. Carnegie Institution of Washington: 1–49. Bibcode:1926CMWCI.324....1H.
  49. ^ Sandage, A. (1989). "Edwin Hubble, 1889–1953". Journal of the Royal Astronomical Society of Canada. 83 (6): 351–362. Bibcode:1989JRASC..83..351S. Archived from the original on May 30, 2012. Retrieved January 8, 2007.
  50. ^ Corbelli, E.; Salucci, P. (2000). "The extended rotation curve and the dark matter halo of M33". Monthly Notices of the Royal Astronomical Society. 311 (2): 441–447. arXiv:astro-ph/9909252. Bibcode:2000MNRAS.311..441C. doi:10.1046/j.1365-8711.2000.03075.x.
  51. ^ Tenn, J. "Hendrik Christoffel van de Hulst". Sonoma State University. Archived from the original on May 29, 2012. Retrieved January 5, 2007.
  52. ^ López-Corredoira, M.; et al. (2001). "Searching for the in-plane Galactic bar and ring in DENIS". Astronomy and Astrophysics. 373 (1): 139–152. arXiv:astro-ph/0104307. Bibcode:2001A&A...373..139L. doi:10.1051/0004-6361:20010560. S2CID 18399375.
  53. ^ Rubin, V. C. (1983). "Dark matter in spiral galaxies". Scientific American. Vol. 248, no. 6. pp. 96–106. Bibcode:1983SciAm.248f..96R. doi:10.1038/scientificamerican0683-96.
  54. ^ Rubin, V. C. (2000). "One Hundred Years of Rotating Galaxies". Publications of the Astronomical Society of the Pacific. 112 (772): 747–750. Bibcode:2000PASP..112..747R. doi:10.1086/316573. S2CID 122927800.
  55. ^ "Hubble Rules Out a Leading Explanation for Dark Matter". Hubble News Desk. October 17, 1994. Archived from the original on August 1, 2012. Retrieved January 8, 2007.
  56. ^ "How many galaxies are there?". NASA. November 27, 2002. Archived from the original on July 11, 2012. Retrieved January 8, 2007.
  57. ^ Kraan-Korteweg, R. C.; Juraszek, S. (2000). "Mapping the hidden Universe: The galaxy distribution in the Zone of Avoidance". Publications of the Astronomical Society of Australia. 17 (1): 6–12. arXiv:astro-ph/9910572. Bibcode:2000PASA...17....6K. doi:10.1071/AS00006. S2CID 17900483.
  58. ^ Conselice, Christopher J.; et al. (2016). "The Evolution of Galaxy Number Density at z < 8 and its Implications". The Astrophysical Journal. 830 (2): 83. arXiv:1607.03909. Bibcode:2016ApJ...830...83C. doi:10.3847/0004-637X/830/2/83. S2CID 17424588.
  59. ^ Fountain, Henry (October 17, 2016). "Two Trillion Galaxies, at the Very Least". The New York Times. from the original on December 31, 2019. Retrieved October 17, 2016.
  60. ^ Lauer, Tod R.; et al. (January 11, 2021). "New Horizons Observations of the Cosmic Optical Background". The Astrophysical Journal. 906 (2): 77. arXiv:2011.03052. Bibcode:2021ApJ...906...77L. doi:10.3847/1538-4357/abc881. hdl:1721.1/133770. ISSN 1538-4357. S2CID 226277978.
  61. ^ "New Horizons spacecraft answers the question: How dark is space?". phys.org. from the original on January 15, 2021. Retrieved January 15, 2021.
  62. ^ Fraknoi et al. 2023, pp. 851–856, 907–915.
  63. ^ Barstow, M. A. (2005). . Leicester University Physics Department. Archived from the original on July 29, 2012. Retrieved June 8, 2006.
  64. ^ Matthews, Thomas A.; Morgan, William W.; Schmidt, Maarten (1964). "A Discussion of Galaxies Indentified [sic] with Radio Sources". The Astrophysical Journal. 140: 35. Bibcode:1964ApJ...140...35M. doi:10.1086/147890.
  65. ^ "Discoveries - Highlights | Tracing the Growth of Galaxies". February 6, 2017.
  66. ^ Tonry, John L. (1987). "Properties of cD Galaxies". Structure and Dynamics of Elliptical Galaxies. Vol. 127. p. 89. Bibcode:1987IAUS..127...89T. doi:10.1007/978-94-009-3971-4_7. ISBN 978-90-277-2586-8. S2CID 117980521.
  67. ^ Tovmassian, Hrant M.; Andernach, Heinz (December 11, 2012). "On the formation of cD galaxies and their parent clusters: Formation of cD galaxies and parent clusters". Monthly Notices of the Royal Astronomical Society. 427 (3): 2047–2056. arXiv:1212.0238. doi:10.1111/j.1365-2966.2012.22044.x.
  68. ^ Bender, Ralf; Kormendy, John; Cornell, Mark E.; Fisher, David B. (June 30, 2015). "Structure and Formation of cD Galaxies: NGC 6166 in Abell 2199". The Astrophysical Journal. 807 (1): 56. arXiv:1411.2598. Bibcode:2015ApJ...807...56B. doi:10.1088/0004-637X/807/1/56. ISSN 1538-4357. This idea was entertained in the heyday of the cooling-flow problem, when we observed large amounts of X-ray-emitting, hot gas in clusters but could not measure temperature profiles. [...] This possibility is now regarded as a non-starter.
  69. ^ Fabian, A. C.; Nulsen, P. E. J. (1977). "Subsonic accretion of cooling gas in clusters of galaxies". Monthly Notices of the Royal Astronomical Society. 180 (3): 479. Bibcode:1977MNRAS.180..479F. doi:10.1093/mnras/180.3.479.
  70. ^ McDonald, M.; McNamara, B. R.; Voit, G. M.; Bayliss, M.; Benson, B. A.; Brodwin, M.; Canning, R. E. A.; Florian, M. K.; Garmire, G. P.; Gaspari, M.; Gladders, M. D.; Hlavacek-Larrondo, J.; Kara, E.; Reichardt, C. L.; Russell, H. R. (November 1, 2019). "Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster". The Astrophysical Journal. 885 (1): 63. arXiv:1904.08942. Bibcode:2019ApJ...885...63M. doi:10.3847/1538-4357/ab464c. ISSN 0004-637X.
  71. ^ "Galactic onion". www.spacetelescope.org. from the original on August 6, 2020. Retrieved May 11, 2015.
  72. ^ Williams, M. J.; Bureau, M.; Cappellari, M. (2010). "Kinematic constraints on the stellar and dark matter content of spiral and S0 galaxies". Monthly Notices of the Royal Astronomical Society. 400 (4): 1665–1689. arXiv:0909.0680. Bibcode:2009MNRAS.400.1665W. doi:10.1111/j.1365-2966.2009.15582.x. S2CID 17940107.
  73. ^ Smith, G. (March 6, 2000). "Galaxies — The Spiral Nebulae". University of California, San Diego Center for Astrophysics & Space Sciences. Archived from the original on July 10, 2012. Retrieved November 30, 2006.
  74. ^ Van den Bergh 1998, p. 17
  75. ^ Obreschkow, D.; Glazebrook, K. (February 28, 2014). "Fundamental Mass-Spin-Morphology Relation of Spiral Galaxies". The Astrophysical Journal. 784 (1): 26. arXiv:1312.4543. Bibcode:2014ApJ...784...26O. doi:10.1088/0004-637X/784/1/26. ISSN 0004-637X.
  76. ^ "Fat or flat: Getting galaxies into shape" March 24, 2021, at the Wayback Machine. phys.org. February 2014
  77. ^ Bertin & Lin 1996, pp. 65–85
  78. ^ Belkora 2003, p. 355
  79. ^ Eskridge, P. B.; Frogel, J. A. (1999). "What is the True Fraction of Barred Spiral Galaxies?". Astrophysics and Space Science. 269/270: 427–430. Bibcode:1999Ap&SS.269..427E. doi:10.1023/A:1017025820201. S2CID 189840251.
  80. ^ Bournaud, F.; Combes, F. (2002). "Gas accretion on spiral galaxies: Bar formation and renewal". Astronomy and Astrophysics. 392 (1): 83–102. arXiv:astro-ph/0206273. Bibcode:2002A&A...392...83B. doi:10.1051/0004-6361:20020920. S2CID 17562844.
  81. ^ Knapen, J. H.; Perez-Ramirez, D.; Laine, S. (2002). "Circumnuclear regions in barred spiral galaxies — II. Relations to host galaxies". Monthly Notices of the Royal Astronomical Society. 337 (3): 808–828. arXiv:astro-ph/0207258. Bibcode:2002MNRAS.337..808K. doi:10.1046/j.1365-8711.2002.05840.x. S2CID 10845683.
  82. ^ Alard, C. (2001). "Another bar in the Bulge". Astronomy and Astrophysics Letters. 379 (2): L44–L47. arXiv:astro-ph/0110491. Bibcode:2001A&A...379L..44A. doi:10.1051/0004-6361:20011487. S2CID 18018228.
  83. ^ Sanders, R. (January 9, 2006). "Milky Way galaxy is warped and vibrating like a drum". UCBerkeley News. from the original on January 18, 2014. Retrieved May 24, 2006.
  84. ^ Bell, G. R.; Levine, S. E. (1997). "Mass of the Milky Way and Dwarf Spheroidal Stream Membership". Bulletin of the American Astronomical Society. 29 (2): 1384. Bibcode:1997AAS...19110806B.
  85. ^ "We Just Discovered a New Type of Colossal Galaxy". Futurism. March 21, 2016. from the original on March 24, 2021. Retrieved March 21, 2016.
  86. ^ Ogle, Patrick M.; Lanz, Lauranne; Nader, Cyril; Helou, George (January 1, 2016). "Superluminous Spiral Galaxies". The Astrophysical Journal. 817 (2): 109. arXiv:1511.00659. Bibcode:2016ApJ...817..109O. doi:10.3847/0004-637X/817/2/109. ISSN 0004-637X. S2CID 35287348.
  87. ^ Gerber, R. A.; Lamb, S. A.; Balsara, D. S. (1994). "Ring Galaxy Evolution as a Function of "Intruder" Mass". Bulletin of the American Astronomical Society. 26: 911. Bibcode:1994AAS...184.3204G.
  88. ^ "ISO unveils the hidden rings of Andromeda" (Press release). European Space Agency. October 14, 1998. Archived from the original on August 28, 1999. Retrieved May 24, 2006.
  89. ^ . Harvard-Smithsonian Center for Astrophysics. May 31, 2004. Archived from the original on September 7, 2006. Retrieved December 6, 2006.
  90. ^ Barstow, M. A. (2005). . University of Leicester. Archived from the original on February 27, 2012. Retrieved December 5, 2006.
  91. ^ Fraknoi et al. 2023, pp. 879.
  92. ^ Pandya, Viraj; Romanowsky, Aaron J.; Laine, Seppo; Brodie, Jean P.; Johnson, Benjamin D.; Glaccum, William; Villaume, Alexa; Cuillandre, Jean-Charles; Gwyn, Stephen; Krick, Jessica; Lasker, Ronald; Martín-Navarro, Ignacio; Martinez-Delgado, David; van Dokkum, Pieter (May 1, 2018). "The Stellar Populations of Two Ultra-diffuse Galaxies from Optical and Near-infrared Photometry". The Astrophysical Journal. 858 (1): 29. arXiv:1711.05272. Bibcode:2018ApJ...858...29P. doi:10.3847/1538-4357/aab498. ISSN 0004-637X.
  93. ^ Overbye, Dennis (January 26, 2024). "What Do You Call a Galaxy Without Stars? - To dark matter and dark energy, add dark galaxies — collections of stars so sparse and faint that they are all but invisible". The New York Times. Archived from the original on January 26, 2024.
  94. ^ van Dokkum, Pieter G.; Abraham, Roberto; Merritt, Allison; Zhang, Jielai; Geha, Marla; Conroy, Charlie (January 2015). "Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster". The Astrophysical Journal Letters. 798 (2): L45. arXiv:1410.8141. Bibcode:2015ApJ...798L..45V. doi:10.1088/2041-8205/798/2/L45. ISSN 2041-8205.
  95. ^ a b Mateo, Mario (1998). "Dwarf Galaxies of the Local Group". Annual Review of Astronomy and Astrophysics. 36 (1): 435–506. arXiv:astro-ph/9810070. Bibcode:1998ARA&A..36..435M. doi:10.1146/annurev.astro.36.1.435. ISSN 0066-4146. S2CID 119333888.
  96. ^ Phillipps, S.; Drinkwater, M. J.; Gregg, M. D.; Jones, J. B. (2001). "Ultracompact Dwarf Galaxies in the Fornax Cluster". The Astrophysical Journal. 560 (1): 201–206. arXiv:astro-ph/0106377. Bibcode:2001ApJ...560..201P. doi:10.1086/322517. S2CID 18297376.
  97. ^ Groshong, K. (April 24, 2006). "Strange satellite galaxies revealed around Milky Way". New Scientist. from the original on July 2, 2015. Retrieved January 10, 2007.
  98. ^ Schirber, M. (August 27, 2008). "No Slimming Down for Dwarf Galaxies". ScienceNOW. from the original on May 30, 2020. Retrieved August 27, 2008.
  99. ^ a b c . University of Maryland Department of Astronomy. Archived from the original on May 9, 2006. Retrieved December 19, 2006.
  100. ^ a b c "Interacting Galaxies". Swinburne University. Archived from the original on July 7, 2012. Retrieved December 19, 2006.
  101. ^ "Happy Sweet Sixteen, Hubble Telescope!". NASA. April 24, 2006. Archived from the original on July 14, 2012. Retrieved August 10, 2006.
  102. ^ a b "Starburst Galaxies". Harvard-Smithsonian Center for Astrophysics. August 29, 2006. from the original on March 16, 2019. Retrieved August 10, 2006.
  103. ^ Kennicutt, R. C. Jr.; et al. (2005). Demographics and Host Galaxies of Starbursts. Starbursts: From 30 Doradus to Lyman Break Galaxies. Springer. p. 187. Bibcode:2005ASSL..329..187K. doi:10.1007/1-4020-3539-X_33. ISBN 978-1-4020-3538-8.
  104. ^ Smith, G. (July 13, 2006). "Starbursts & Colliding Galaxies". University of California, San Diego Center for Astrophysics & Space Sciences. Archived from the original on July 7, 2012. Retrieved August 10, 2006.
  105. ^ Keel, B. (September 2006). "Starburst Galaxies". University of Alabama. Archived from the original on June 4, 2012. Retrieved December 11, 2006.
  106. ^ Adams, David J.; Adams, David John; Cayless, Alan; Jones, Anthony W. (2004). Jones, Mark H.; Adams, David J.; Lambourne, Robert J. (eds.). An Introduction to Galaxies and Cosmology. Cambridge University Press. pp. 142–144. ISBN 978-0-521-54623-2.
  107. ^ Kembhavi, Ajit K.; Narlikar, Jayant V. (2012) [1999]. "RADIO PROPERTIES". Quasars and Active Galactic Nuclei: An Introduction. Cambridge: Cambridge University Press. pp. 214–272. ISBN 978-1-139-17440-4.
  108. ^ . Department of Astronomy, University of Maryland. Archived from the original on September 1, 2022. Retrieved September 1, 2022.
  109. ^ Oei, Martijn S. S. L.; van Weeren, Reinout J.; Hardcastle, Martin J.; Botteon, Andrea; Shimwell, Tim W.; Dabhade, Pratik; Gast, Aivin R. D. J. G. I. B.; Röttgering, Huub J. A.; Brüggen, Marcus; Tasse, Cyril; Williams, Wendy L.; Shulevski, Aleksandar (April 1, 2022). "The discovery of a radio galaxy of at least 5 Mpc". Astronomy & Astrophysics. 660: A2. arXiv:2202.05427. Bibcode:2022A&A...660A...2O. doi:10.1051/0004-6361/202142778. ISSN 0004-6361.
  110. ^ Condon, J. J. (September 1992). "Radio Emission from Normal Galaxies". Annual Review of Astronomy and Astrophysics. 30 (1): 575–611. Bibcode:1992ARA&A..30..575C. doi:10.1146/annurev.aa.30.090192.003043. ISSN 0066-4146.
  111. ^ Fraknoi et al. 2023, p. 906.
  112. ^ Zakamska, Nadia L.; Strauss, Michael A.; Krolik, Julian H.; Collinge, Matthew J.; Hall, Patrick B.; Hao, Lei; Heckman, Timothy M.; Ivezi, eljko; Richards, Gordon T.; Schlegel, David J.; Schneider, Donald P.; Strateva, Iskra; Vanden Berk, Daniel E.; Anderson, Scott F.; Brinkmann, Jon (November 2003). "Candidate Type II Quasars from the Sloan Digital Sky Survey. I. Selection and Optical Properties of a Sample at 0.3< Z". The Astronomical Journal. 126 (5): 2125–2144. arXiv:astro-ph/0309551. doi:10.1086/378610. ISSN 0004-6256.
  113. ^ Fraknoi et al. 2023, pp. 907–915.
  114. ^ a b Keel, W. C. (2000). "Introducing Active Galactic Nuclei". University of Alabama. Archived from the original on July 27, 2012. Retrieved December 6, 2006.
  115. ^ a b Lochner, J.; Gibb, M. "A Monster in the Middle". NASA. Archived from the original on July 10, 2012. Retrieved December 20, 2006.
  116. ^ Peterson, Bradley M. (1997). An Introduction to Active Galactic Nuclei. Cambridge University Press. ISBN 978-0-521-47911-0.
  117. ^ Peterson, Bradley M. (August 1, 2008). "The central black hole and relationships with the host galaxy" (PDF). New Astronomy Reviews. Active Galactic Nuclei at the Highest Angular Resolution: Theory and Observations. 52 (6): 240–252. Bibcode:2008NewAR..52..240P. doi:10.1016/j.newar.2008.06.005. ISSN 1387-6473. S2CID 121460317.
  118. ^ Fraknoi et al. 2023, pp. 899–906.
  119. ^ Kovačević, Andjelka B; Yi, Tignfeng; Dai, Xinyu; Yang, Xing; Čvorović-Hajdinjak, Iva; Popović, Luka Č (May 21, 2020). "Confirmed short periodic variability of subparsec supermassive binary black hole candidate Mrk 231". Monthly Notices of the Royal Astronomical Society. 494 (3): 4069–4076. arXiv:2003.06359. doi:10.1093/mnras/staa737. ISSN 0035-8711.
  120. ^ "UHZ1: Distant Galaxy and Black Hole". Astronomy Picture of the Day. November 10, 2023. Retrieved February 18, 2024.
  121. ^ Bogdán, Ákos; Goulding, Andy D.; Natarajan, Priyamvada; Kovács, Orsolya E.; Tremblay, Grant R.; Chadayammuri, Urmila; Volonteri, Marta; Kraft, Ralph P.; Forman, William R.; Jones, Christine; Churazov, Eugene; Zhuravleva, Irina (January 2024). "Evidence for heavy-seed origin of early supermassive black holes from a z ≈ 10 X-ray quasar". Nature Astronomy. 8 (1): 126–133. arXiv:2305.15458. Bibcode:2024NatAs...8..126B. doi:10.1038/s41550-023-02111-9. ISSN 2397-3366. S2CID 258887541.
  122. ^ Blandford, R. D.; Narayan, R. (September 1992). "Cosmological Applications of Gravitational Lensing". Annual Review of Astronomy and Astrophysics. 30 (1): 311–358. doi:10.1146/annurev.aa.30.090192.001523. ISSN 0066-4146.
  123. ^ a b Heckman, T. M. (1980). "An optical and radio survey of the nuclei of bright galaxies — Activity in normal galactic nuclei". Astronomy and Astrophysics. 87: 152–164. Bibcode:1980A&A....87..152H.
  124. ^ Ho, L. C.; Filippenko, A. V.; Sargent, W. L. W. (1997). "A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity in Nearby Galaxies". The Astrophysical Journal. 487 (2): 568–578. arXiv:astro-ph/9704108. Bibcode:1997ApJ...487..568H. doi:10.1086/304638. S2CID 16742031.
  125. ^ Pérez-Torres, Miguel; Mattila, Seppo; Alonso-Herrero, Almudena; Aalto, Susanne; Efstathiou, Andreas (December 2021). "Star formation and nuclear activity in luminous infrared galaxies: an infrared through radio review". The Astronomy and Astrophysics Review. 29 (1): 2. arXiv:2010.05072. Bibcode:2021A&ARv..29....2P. doi:10.1007/s00159-020-00128-x. ISSN 0935-4956. S2CID 253687419.
  126. ^ a b Bellocchi, E.; Pereira-Santaella, M.; Colina, L.; Labiano, A.; Sánchez-García, M.; Alonso-Herrero, A.; Arribas, S.; García-Burillo, S.; Villar-Martín, M.; Rigopoulou, D.; Valentino, F.; Puglisi, A.; Díaz-Santos, T.; Cazzoli, S.; Usero, A. (August 2022). "Compact molecular gas emission in local LIRGs among low- and high- z galaxies". Astronomy & Astrophysics. 664: A60. arXiv:2204.02055. Bibcode:2022A&A...664A..60B. doi:10.1051/0004-6361/202142802. ISSN 0004-6361.
  127. ^ Sanders, D. B.; Mirabel, I. F. (September 1996). "Luminous Infrared Galaxies" (PDF). Annual Review of Astronomy and Astrophysics. 34 (1): 749–792. Bibcode:1996ARA&A..34..749S. doi:10.1146/annurev.astro.34.1.749. ISSN 0066-4146.
  128. ^ a b Sandage, A. R., Kron, R. G., & Longair, M. S. (1995). The Deep Universe: Saas-Fee Advanced Course 23. Lecture Notes 1993. Swiss Society for Astrophysics and Astronomy (Vol. 23). Springer Science & Business Media.
  129. ^ a b Chamba, Nushkia (2020). "A historical perspective on the concept of galaxy size". Research Notes of the American Astronomical Society. 4 (7): 117. arXiv:2010.07946. Bibcode:2020RNAAS...4..117C. doi:10.3847/2515-5172/aba951. S2CID 223953458.
  130. ^ Mushotzky, Richard. "Measuring Galaxy Luminosities" (PDF). ASTR620: Galaxies. University of Maryland Astronomy Department. Retrieved February 14, 2024.
  131. ^ Sparke & Gallagher 2000, 1.3.1
  132. ^ Fouque, P.; Paturel, G. (September 1985). "Standard photometric diameters of galaxies. II - Reduction of the ESO, UGC, MCG catalogue". Astronomy and Astrophysics. 150 (2): 192–204. Bibcode:1985A&A...150..192F.
  133. ^ "By Name | NASA/IPAC Extragalactic Database". ned.ipac.caltech.edu. Retrieved March 8, 2024.
  134. ^ Goodwin, S. P., John Gribbin, and M. A. Hendry. "The relative size of the Milky Way." The Observatory 118 (1998): 201-208.
  135. ^ "By Name | NASA/IPAC Extragalactic Database". ned.ipac.caltech.edu. Retrieved March 8, 2024.
  136. ^ "By Name | NASA/IPAC Extragalactic Database". ned.ipac.caltech.edu. Retrieved March 8, 2024.
  137. ^ De Vaucouleurs, Gerard (1948). "Recherches sur les Nebuleuses Extragalactiques". Annales d'Astrophysique. 11: 247. Bibcode:1948AnAp...11..247D.
  138. ^ Fish, Robert A. (1963). "Significance of a Luminosity Concentration Law in Elliptical Galaxies". The Astronomical Journal. 68: 72. Bibcode:1963AJ.....68R..72F. doi:10.1086/109075.
  139. ^ Sérsic, J. L. (1968). "On the formation of galaxies by fragmentation". Bulletin of the Astronomical Institutes of Czechoslovakia. 19: 105. Bibcode:1968BAICz..19..105S.
  140. ^ Conselice, Christopher J.; Bershady, Matthew A.; Jangren, Anna (2000). "The Asymmetry of Galaxies: Physical Morphology for Nearby and High-Redshift Galaxies". The Astrophysical Journal. 529 (2): 886–910. arXiv:astro-ph/9907399. Bibcode:2000ApJ...529..886C. doi:10.1086/308300. S2CID 118962524.
  141. ^ Burgarella, D.; Buat, V.; Donas, J.; Milliard, B.; Chapelon, S. (2001). "The ultraviolet visibility and quantitative morphology of galactic disks at low and high redshift". Astronomy & Astrophysics. 369 (2): 421–431. arXiv:astro-ph/0101344. Bibcode:2001A&A...369..421B. doi:10.1051/0004-6361:20010107. S2CID 858029.
  142. ^ Lauberts, Andris; Valentijn, Edwin A. (1989). The surface photometry catalogue of the ESO-Uppsala galaxies. Bibcode:1989spce.book.....L.
  143. ^ Petrosian, V. (1976). "Surface Brightness and Evolution of Galaxies". The Astrophysical Journal. 210: L53. Bibcode:1976ApJ...209L...1P. doi:10.1086/182301.
  144. ^ "SkyServer: Algorithms".
  145. ^ a b Graham, Alister W.; Driver, Simon P.; Petrosian, Vahé; Conselice, Christopher J.; Bershady, Matthew A.; Crawford, Steven M.; Goto, Tomotsugu (2005). "Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii". The Astronomical Journal. 130 (4): 1535–1544. arXiv:astro-ph/0504287. Bibcode:2005AJ....130.1535G. doi:10.1086/444475. S2CID 11517686.
  146. ^ "Circular and Fixed Elliptical Apertures: The Petrosian and Isophotal Photometry".
  147. ^ "About NED | NASA/IPAC Extragalactic Database". ned.ipac.caltech.edu.
  148. ^ a b Jarrett, T. H.; Chester, T.; Cutri, R.; Schneider, S. E.; Huchra, J. P. (2003). "The 2MASS Large Galaxy Atlas". The Astronomical Journal. 125 (2): 525–554. Bibcode:2003AJ....125..525J. doi:10.1086/345794. S2CID 117784410.
  149. ^ a b Beck, Rainer (2007). "Galactic magnetic fields". Scholarpedia. 2 (8): 2411. Bibcode:2007SchpJ...2.2411B. doi:10.4249/scholarpedia.2411.
  150. ^ "Construction Secrets of a Galactic Metropolis". www.eso.org. ESO Press Release. from the original on March 24, 2021. Retrieved October 15, 2014.
  151. ^ a b . Harvard-Smithsonian Center for Astrophysics. November 18, 1999. Archived from the original on March 25, 2008. Retrieved January 10, 2007.
  152. ^ Firmani, C.; Avila-Reese, V. (2003). "Physical processes behind the morphological Hubble sequence". Revista Mexicana de Astronomía y Astrofísica. 17: 107–120. arXiv:astro-ph/0303543. Bibcode:2003RMxAC..17..107F.
  153. ^ Skibba, Ramin (January 10, 2023). "Astronomers May Have Just Spotted the Universe's First Galaxies - NASA's new JWST space telescope has revealed some cosmic surprises, including galaxies that might have assembled earlier than previously thought". Wired. Retrieved January 10, 2023.
  154. ^ McMahon, R. (2006). "Astronomy: Dawn after the dark age". Nature. 443 (7108): 151–2. Bibcode:2006Natur.443..151M. doi:10.1038/443151a. PMID 16971933. S2CID 28977650.
  155. ^ Wall, Mike (December 12, 2012). "Ancient Galaxy May Be Most Distant Ever Seen". Space.com. from the original on May 4, 2019. Retrieved December 12, 2012.
  156. ^ "Cosmic Detectives". The European Space Agency (ESA). April 2, 2013. from the original on February 11, 2019. Retrieved April 15, 2013.
  157. ^ "HubbleSite – NewsCenter – Astronomers Set a New Galaxy Distance Record (05/05/2015) – Introduction". hubblesite.org. from the original on December 9, 2016. Retrieved May 7, 2015.
  158. ^ "This Galaxy Far, Far Away Is the Farthest One Yet Found". Space.com. May 5, 2015. from the original on October 2, 2015. Retrieved May 7, 2015.
  159. ^ "Astronomers unveil the farthest galaxy". from the original on September 11, 2017. Retrieved May 7, 2015.
  160. ^ Overbye, Dennis (May 5, 2015). "Astronomers Measure Distance to Farthest Galaxy Yet". The New York Times. ISSN 0362-4331. from the original on April 13, 2019. Retrieved May 7, 2015.
  161. ^ Oesch, P. A.; van Dokkum, P. G.; Illingworth, G. D.; Bouwens, R. J.; Momcheva, I.; Holden, B.; Roberts-Borsani, G. W.; Smit, R.; Franx, M. (February 18, 2015). "A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z=7.730 using Keck/MOSFIRE". The Astrophysical Journal. 804 (2): L30. arXiv:1502.05399. Bibcode:2015ApJ...804L..30O. doi:10.1088/2041-8205/804/2/L30. S2CID 55115344.
  162. ^ Atkinson, Nancy (August 17, 2022). "Here's the Largest Image JWST Has Taken So Far". Universe Today. Retrieved August 18, 2022.
  163. ^ Whitt, Kelly Kizer (August 18, 2022). "Webb's largest image of galaxies yet". Earth & Sky. Retrieved August 19, 2022.
  164. ^ Overbye, Dennis (August 23, 2022). "How the Webb Telescope Expanded My Universe - As new images of Jupiter and a galactic survey spring forth from NASA's new observatory, our cosmic affairs correspondent confesses he didn't anticipate their power". The New York Times. Retrieved August 24, 2022.
  165. ^ a b c d Roberts-Borsani, G.; Treu, T.; Chen, W. (May 17, 2023). "The nature of an ultra-faint galaxy in the cosmic dark ages seen with JWST". Nature. 618 (7965): 480–483. arXiv:2210.15639. Bibcode:2023Natur.618..480R. doi:10.1038/s41586-023-05994-w. PMID 37198479. S2CID 258741077.
  166. ^ a b c Ober, Holly (June 1, 2023). "Astrophysicists confirm the faintest galaxy ever seen in the early universe". Phys.org. Retrieved June 12, 2023.
  167. ^ "Signatures of the Earliest Galaxies". from the original on August 6, 2020. Retrieved September 15, 2015.
  168. ^ Eggen, O. J.; Lynden-Bell, D.; Sandage, A. R. (1962). "Evidence from the motions of old stars that the Galaxy collapsed". The Astrophysical Journal. 136: 748. Bibcode:1962ApJ...136..748E. doi:10.1086/147433.
  169. ^ Searle, L.; Zinn, R. (1978). "Compositions of halo clusters and the formation of the galactic halo". The Astrophysical Journal. 225 (1): 357–379. Bibcode:1978ApJ...225..357S. doi:10.1086/156499.
  170. ^ Heger, A.; Woosley, S. E. (2002). "The Nucleosynthetic Signature of Population III". The Astrophysical Journal. 567 (1): 532–543. arXiv:astro-ph/0107037. Bibcode:2002ApJ...567..532H. doi:10.1086/338487. S2CID 16050642.
  171. ^ Barkana, R.; Loeb, A. (2001). "In the beginning: the first sources of light and the reionization of the Universe" (PDF). Physics Reports (Submitted manuscript). 349 (2): 125–238. arXiv:astro-ph/0010468. Bibcode:2001PhR...349..125B. doi:10.1016/S0370-1573(01)00019-9. S2CID 119094218. (PDF) from the original on March 14, 2021. Retrieved July 25, 2018.
  172. ^ Sobral, David; Matthee, Jorryt; Darvish, Behnam; Schaerer, Daniel; Mobasher, Bahram; Röttgering, Huub J. A.; Santos, Sérgio; Hemmati, Shoubaneh (June 4, 2015). "Evidence for POPIII-like Stellar Populations in the Most Luminous LYMAN-α Emitters at the Epoch of Re-ionisation: Spectroscopic Confirmation". The Astrophysical Journal. 808 (2): 139. arXiv:1504.01734. Bibcode:2015ApJ...808..139S. doi:10.1088/0004-637x/808/2/139. S2CID 18471887.
  173. ^ Overbye, Dennis (June 17, 2015). "Traces of Earliest Stars That Enriched Cosmos Are Spied". The New York Times. from the original on June 29, 2019. Retrieved June 17, 2015.
  174. ^ Overbye, Dennis (January 5, 2024). "The Early Universe Was Bananas - Images from the Webb telescope suggest that newborn galaxies look weirder than expected. Exactly how screwy was physics at the dawn of time?". The New York Times. Archived from the original on January 5, 2024. Retrieved January 6, 2024.
  175. ^ Pandya, Viraj; et al. (October 23, 2023). "Galaxies Going Bananas: Inferring the 3D Geometry of High-Redshift Galaxies with JWST-CEERS". arXiv:2310.15232 [astro-ph.GA].
  176. ^ O'Callaghan, Jonathan (December 6, 2022). "Astronomers Grapple with JWST's Discovery of Early Galaxies - Researchers are convinced the James Webb Space Telescope has glimpsed an unexpected population of galaxies in the early universe. Now they're trying to decide what this means for our understanding of the cosmos". Scientific American. Retrieved December 6, 2022.
  177. ^ "Simulations Show How Growing Black Holes Regulate Galaxy Formation". Carnegie Mellon University. February 9, 2005. Archived from the original on June 4, 2012. Retrieved January 7, 2007.
  178. ^ Massey, R. (April 21, 2007). . Royal Astronomical Society. Archived from the original on November 15, 2013. Retrieved April 20, 2007.
  179. ^ Noguchi, M. (1999). "Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks". The Astrophysical Journal. 514 (1): 77–95. arXiv:astro-ph/9806355. Bibcode:1999ApJ...514...77N. doi:10.1086/306932. S2CID 17963236.
  180. ^ Baugh, C.; Frenk, C. (May 1999). . PhysicsWeb. Archived from the original on April 26, 2007. Retrieved January 16, 2007.
  181. ^ Gonzalez, G. (1998). The Stellar Metallicity — Planet Connection. Brown dwarfs and extrasolar planets: Proceedings of a workshop ... p. 431. Bibcode:1998ASPC..134..431G.
  182. ^ Moskowitz, Clara (September 25, 2012). "Hubble Telescope Reveals Farthest View Into Universe Ever". Space.com. from the original on May 5, 2020. Retrieved September 26, 2012.
  183. ^ Rojas, Randall R.; Vogeley, Michael S.; Hoyle, Fiona; Brinkmann, Jon (May 10, 2005). "Spectroscopic Properties of Void Galaxies in the Sloan Digital Sky Survey". The Astrophysical Journal. 624 (2): 571–585. arXiv:astro-ph/0409074. doi:10.1086/428476. ISSN 0004-637X.
  184. ^ Conselice, C. J. (February 2007). "The Universe's Invisible Hand". Scientific American. Vol. 296, no. 2. pp. 35–41. Bibcode:2007SciAm.296b..34C. doi:10.1038/scientificamerican0207-34.
  185. ^ Ford, H.; et al. (April 30, 2002). "The Mice (NGC 4676): Colliding Galaxies With Tails of Stars and Gas". Hubble News Desk. from the original on September 7, 2016. Retrieved May 8, 2007.
  186. ^ Struck, C. (1999). "Galaxy Collisions". Physics Reports. 321 (1–3): 1–137. arXiv:astro-ph/9908269. Bibcode:1999PhR...321....1S. doi:10.1016/S0370-1573(99)00030-7. S2CID 119369136.
  187. ^ Buser, R. (2000). "The Formation and Early Evolution of the Milky Way Galaxy". Science. 287 (5450): 69–74. Bibcode:2000Sci...287...69B. doi:10.1126/science.287.5450.69. PMID 10615051.
  188. ^ Kruijssen, J M Diederik; Pfeffer, Joel L; Chevance, Mélanie; Bonaca, Ana; Trujillo-Gomez, Sebastian; Bastian, Nate; Reina-Campos, Marta; Crain, Robert A; Hughes, Meghan E (October 2020). "Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations". Monthly Notices of the Royal Astronomical Society. 498 (2): 2472–2491. arXiv:2003.01119. doi:10.1093/mnras/staa2452. from the original on November 16, 2020. Retrieved November 15, 2020.
  189. ^ Young, Monica (November 13, 2020). "Star Clusters reveal the "Kraken" in the Milky Way's Past". Sky and Telescope. from the original on November 15, 2020. Retrieved November 15, 2020.
  190. ^ Panter, B.; Jimenez, R.; Heavens, A. F.; Charlot, S. (2007). "The star formation histories of galaxies in the Sloan Digital Sky Survey". Monthly Notices of the Royal Astronomical Society. 378 (4): 1550–1564. arXiv:astro-ph/0608531. Bibcode:2007MNRAS.378.1550P. doi:10.1111/j.1365-2966.2007.11909.x. S2CID 15174718.
  191. ^ Kennicutt, R. C. Jr.; Tamblyn, P.; Congdon, C. E. (1994). "Past and future star formation in disk galaxies". The Astrophysical Journal. 435 (1): 22–36. Bibcode:1994ApJ...435...22K. doi:10.1086/174790.
  192. ^ Knapp, G. R. (1999). Star Formation in Early Type Galaxies. Vol. 163. Astronomical Society of the Pacific. p. 119. arXiv:astro-ph/9808266. Bibcode:1999ASPC..163..119K. ISBN 978-1-886733-84-8. OCLC 41302839. from the original on March 24, 2021. Retrieved July 25, 2018.
  193. ^ a b Adams, Fred; Laughlin, Greg (July 13, 2006). "The Great Cosmic Battle". Astronomical Society of the Pacific. Archived from the original on July 31, 2012. Retrieved January 16, 2007.
  194. ^ "Cosmic 'Murder Mystery' Solved: Galaxies Are 'Strangled to Death'". Space.com. May 13, 2015. from the original on March 24, 2021. Retrieved May 14, 2015.
  195. ^ Pobojewski, S. (January 21, 1997). "Physics offers glimpse into the dark side of the Universe". University of Michigan. Archived from the original on June 4, 2012. Retrieved January 13, 2007.
  196. ^ "Groups & Clusters of Galaxies". NASA/Chandra. Archived from the original on July 7, 2012. Retrieved January 15, 2007.
  197. ^ Ricker, P. "When Galaxy Clusters Collide". San Diego Supercomputer Center. Archived from the original on August 5, 2012. Retrieved August 27, 2008.
  198. ^ Dahlem, M. (November 24, 2006). . University of Birmingham Astrophysics and Space Research Group. Archived from the original on June 13, 2007. Retrieved January 15, 2007.
  199. ^ Ponman, T. (February 25, 2005). . University of Birmingham Astrophysics and Space Research Group. Archived from the original on February 15, 2009. Retrieved January 15, 2007.
  200. ^ Girardi, M.; Giuricin, G. (2000). "The Observational Mass Function of Loose Galaxy Groups". The Astrophysical Journal. 540 (1): 45–56. arXiv:astro-ph/0004149. Bibcode:2000ApJ...540...45G. doi:10.1086/309314. S2CID 14059401.
  201. ^ "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen". ESA/Hubble Press Release. from the original on June 12, 2018. Retrieved January 22, 2015.
  202. ^ Dubinski, J. (1998). . The Astrophysical Journal. 502 (2): 141–149. arXiv:astro-ph/9709102. Bibcode:1998ApJ...502..141D. doi:10.1086/305901. S2CID 3137328. Archived from the original on May 14, 2011. Retrieved January 16, 2007.
  203. ^ "ATLASGAL Survey of Milky Way Completed". from the original on March 24, 2021. Retrieved March 7, 2016.
  204. ^ Bahcall, N. A. (1988). "Large-scale structure in the Universe indicated by galaxy clusters". Annual Review of Astronomy and Astrophysics. 26 (1): 631–686. Bibcode:1988ARA&A..26..631B. doi:10.1146/annurev.aa.26.090188.003215.
  205. ^ Mandolesi, N.; et al. (1986). "Large-scale homogeneity of the Universe measured by the microwave background". Letters to Nature. 319 (6056): 751–753. Bibcode:1986Natur.319..751M. doi:10.1038/319751a0. S2CID 4349689.
  206. ^ Horváth, István; Bagoly, Zsolt; Hakkila, Jon; Tóth, L. Viktor (2015). "New data support the existence of the Hercules–Corona Borealis Great Wall". Astronomy & Astrophysics. 584: A48. arXiv:1510.01933. Bibcode:2015A&A...584A..48H. doi:10.1051/0004-6361/201424829. S2CID 56073380.
  207. ^ Horváth, István; Bagoly, Zsolt; Hakkila, Jon; Tóth, L. Viktor (2014). "Anomalies in the GRB spatial distribution". Proceedings of Science: 78. arXiv:1507.05528. Bibcode:2014styd.confE..78H. doi:10.22323/1.233.0078.
  208. ^ van den Bergh, S. (2000). "Updated Information on the Local Group". Publications of the Astronomical Society of the Pacific. 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548. S2CID 1805423.
  209. ^ Tully, R. B. (1982). "The Local Supercluster". The Astrophysical Journal. 257: 389–422. Bibcode:1982ApJ...257..389T. doi:10.1086/159999.
  210. ^ Tempel, Elmo (September 1, 2014). "Cosmology: Meet the Laniakea supercluster". Nature. 513 (7516): 41–42. Bibcode:2014Natur.513...41T. doi:10.1038/513041a. PMID 25186896.
  211. ^ Rieke, George Henry (2012). Measuring the universe: a multiwavelength perspective. Cambridge: Cambridge university press. ISBN 978-0-521-76229-8.
  212. ^ . IPAC/NASA. Archived from the original on December 30, 2006. Retrieved January 2, 2007.
  213. ^ Fraknoi et al. 2023, pp. 144, 202.
  214. ^ "The Effects of Earth's Upper Atmosphere on Radio Signals". NASA. Archived from the original on May 29, 2012. Retrieved August 10, 2006.
  215. ^ Max Planck Society (December 14, 2006). "Giant Radio Telescope Imaging Could Make Dark Matter Visible". ScienceDaily. from the original on July 3, 2017. Retrieved January 2, 2007.
  216. ^ "NASA Telescope Sees Black Hole Munch on a Star". NASA. December 5, 2006. Archived from the original on June 4, 2012. Retrieved January 2, 2007.
  217. ^ Dunn, R. (August 9, 2005). "An Introduction to X-ray Astronomy". Institute of Astronomy X-Ray Group. Archived from the original on July 17, 2012. Retrieved January 2, 2007.
  218. ^ "Webb reveals structure in 19 spiral galaxies". ESA. January 29, 2024. Retrieved January 30, 2024.

Bibliography

External links

  • NASA/IPAC Extragalactic Database (NED) (NED-Distances)
  • Galaxies on In Our Time at the BBC
  • Citizen science galaxy classification project; see Galaxy zoo
  • How many galaxies are in our universe? August 21, 2015, at the Wayback Machine
  • "A Flight Through the Universe, by the Sloan Digital Sky Survey" – animated video from Berkely Lab

April 10, 2024
galaxy, this, article, about, astronomical, structure, earth, galaxy, milky, other, uses, disambiguation, galaxy, system, stars, stellar, remnants, interstellar, dust, dark, matter, bound, together, gravity, word, derived, from, greek, galaxias, γαλαξίας, lite. This article is about the astronomical structure For Earth s galaxy see Milky Way For other uses see Galaxy disambiguation A galaxy is a system of stars stellar remnants interstellar gas dust and dark matter bound together by gravity 1 2 The word is derived from the Greek galaxias gala3ias literally milky a reference to the Milky Way galaxy that contains the Solar System Galaxies averaging an estimated 100 million stars 3 range in size from dwarfs with less than a thousand stars 4 to the largest galaxies known supergiants with one hundred trillion stars each orbiting its galaxy s center of mass Most of the mass in a typical galaxy is in the form of dark matter with only a few percent of that mass visible in the form of stars and nebulae Supermassive black holes are a common feature at the centres of galaxies NGC 4414 a typical spiral galaxy in the constellation Coma Berenices is about 55 000 light years in diameter and approximately 60 million light years from Earth Galaxies are categorized according to their visual morphology as elliptical 5 spiral or irregular 6 Many are thought to have supermassive black holes at their centers The Milky Way s central black hole known as Sagittarius A has a mass four million times greater than the Sun 7 It is estimated that there are between 200 billion 8 2 1011 to 2 trillion 9 galaxies in the observable universe Most galaxies are 1 000 to 100 000 parsecs in diameter approximately 3 000 to 300 000 light years and are separated by distances on the order of millions of parsecs or megaparsecs For comparison the Milky Way has a diameter of at least 26 800 parsecs 87 400 ly and is separated from the Andromeda Galaxy with diameter of about 152 000 ly its nearest large neighbor by 780 000 parsecs 2 5 million ly The space between galaxies is filled with a tenuous gas the intergalactic medium with an average density of less than one atom per cubic meter Most galaxies are gravitationally organized into groups clusters and superclusters The Milky Way is part of the Local Group which it dominates along with the Andromeda Galaxy The group is part of the Virgo Supercluster At the largest scale these associations are generally arranged into sheets and filaments surrounded by immense voids 10 Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea 11 Contents 1 Etymology 2 Nomenclature 3 Observation history 3 1 Milky Way 3 2 Distinction from other nebulae 3 3 Modern research 4 Types and morphology 4 1 Ellipticals 4 1 1 Type cD galaxies 4 1 2 Shell galaxy 4 2 Spirals 4 2 1 Barred spiral galaxy 4 2 2 Super luminous spiral 4 3 Other morphologies 4 4 Dwarfs 5 Variants 5 1 Interacting 5 2 Starburst 5 3 Radio galaxy 5 4 Active galaxy 5 4 1 Seyfert galaxy 5 4 2 Quasar 5 4 3 Other AGNs 5 5 Luminous infrared galaxy 6 Physical diameters 6 1 Angular diameter 6 2 Isophotal diameter 6 3 Effective radius half light and its variations 6 4 Petrosian magnitude 6 5 Near infrared method 7 Magnetic fields 8 Formation and evolution 8 1 Formation 8 1 1 Early galaxy formation 8 2 Evolution 8 3 Future trends 9 Larger scale structures 10 Multi wavelength observation 11 Gallery 12 See also 13 Notes 14 References 15 Bibliography 16 External linksEtymologyThe word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way galaxias kuklos gala3ias kyklos 12 13 milky circle named after its appearance as a milky band of light in the sky In Greek mythology Zeus places his son born by a mortal woman the infant Heracles on Hera s breast while she is asleep so the baby will drink her divine milk and thus become immortal Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby she pushes the baby away some of her milk spills and it produces the band of light known as the Milky Way 14 15 In the astronomical literature the capitalized word Galaxy is often used to refer to the Milky Way galaxy to distinguish it from the other galaxies in the observable universe The English term Milky Way can be traced back to a story by Geoffrey Chaucer c 1380 See yonder lo the Galaxye Which men clepeth the Milky Wey For hit is whyt Geoffrey Chaucer The House of Fame 13 Galaxies were initially discovered telescopically and were known as spiral nebulae Most 18th to 19th century astronomers considered them as either unresolved star clusters or anagalactic nebulae and were just thought of as a part of the Milky Way but their true composition and natures remained a mystery Observations using larger telescopes of a few nearby bright galaxies like the Andromeda Galaxy began resolving them into huge conglomerations of stars but based simply on the apparent faintness and sheer population of stars the true distances of these objects placed them well beyond the Milky Way For this reason they were popularly called island universes but this term quickly fell into disuse as the word universe implied the entirety of existence Instead they became known simply as galaxies 16 Nomenclature nbsp Galaxy cluster SDSS J1152 3313 SDSS stands for Sloan Digital Sky Survey J for Julian epoch and 1152 3313 for right ascension and declination respectively Millions of galaxies have been catalogued but only a few have well established names such as the Andromeda Galaxy the Magellanic Clouds the Whirlpool Galaxy and the Sombrero Galaxy Astronomers work with numbers from certain catalogues such as the Messier catalogue the NGC New General Catalogue the IC Index Catalogue the CGCG Catalogue of Galaxies and of Clusters of Galaxies the MCG Morphological Catalogue of Galaxies the UGC Uppsala General Catalogue of Galaxies and the PGC Catalogue of Principal Galaxies also known as LEDA All the well known galaxies appear in one or more of these catalogs but each time under a different number For example Messier 109 or M109 is a spiral galaxy having the number 109 in the catalog of Messier It also has the designations NGC 3992 UGC 6937 CGCG 269 023 MCG 09 20 044 and PGC 37617 or LEDA 37617 among others 17 Millions of fainter galaxies are known by their identifiers in sky surveys such as the Sloan Digital Sky Survey 18 Observation historyMilky Way Main article Milky Way Greek philosopher Democritus 450 370 BCE proposed that the bright band on the night sky known as the Milky Way might consist of distant stars 19 Aristotle 384 322 BCE however believed the Milky Way was caused by the ignition of the fiery exhalation of some stars that were large numerous and close together and that the ignition takes place in the upper part of the atmosphere in the region of the World that is continuous with the heavenly motions 20 Neoplatonist philosopher Olympiodorus the Younger c 495 570 CE was critical of this view arguing that if the Milky Way was sublunary situated between Earth and the Moon it should appear different at different times and places on Earth and that it should have parallax which it did not In his view the Milky Way was celestial 21 According to Mohani Mohamed Arabian astronomer Ibn al Haytham 965 1037 made the first attempt at observing and measuring the Milky Way s parallax 22 and he thus determined that because the Milky Way had no parallax it must be remote from the Earth not belonging to the atmosphere 23 Persian astronomer al Biruni 973 1048 proposed the Milky Way galaxy was a collection of countless fragments of the nature of nebulous stars 24 Andalusian astronomer Avempace d 1138 proposed that it was composed of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material 20 25 citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near 20 In the 14th century Syrian born Ibn Qayyim al Jawziyya proposed the Milky Way galaxy was a myriad of tiny stars packed together in the sphere of the fixed stars 26 Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed of a huge number of faint stars 27 28 In 1750 English astronomer Thomas Wright in his An Original Theory or New Hypothesis of the Universe correctly speculated that it might be a rotating body of a huge number of stars held together by gravitational forces akin to the Solar System but on a much larger scale and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it 29 30 In his 1755 treatise Immanuel Kant elaborated on Wright s idea about the Milky Way s structure 31 nbsp The shape of the Milky Way as estimated from star counts by William Herschel in 1785 the Solar System was assumed to be near the center The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky He produced a diagram of the shape of the galaxy with the Solar System close to the center 32 33 Using a refined approach Kapteyn in 1920 arrived at the picture of a small diameter about 15 kiloparsecs ellipsoid galaxy with the Sun close to the center A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center 30 Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters the present picture of the Milky Way galaxy emerged 34 Distinction from other nebulae A few galaxies outside the Milky Way are visible on a dark night to the unaided eye including the Andromeda Galaxy Large Magellanic Cloud Small Magellanic Cloud and the Triangulum Galaxy In the 10th century Persian astronomer Abd al Rahman al Sufi made the earliest recorded identification of the Andromeda Galaxy describing it as a small cloud 35 In 964 he probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars referring to Al Bakr of the southern Arabs 36 since at a declination of about 70 south it was not visible where he lived it was not well known to Europeans until Magellan s voyage in the 16th century 37 36 The Andromeda Galaxy was later independently noted by Simon Marius in 1612 35 In 1734 philosopher Emanuel Swedenborg in his Principia speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what could be seen These views are remarkably close to the present day views of the cosmos 38 In 1745 Pierre Louis Maupertuis conjectured that some nebula like objects were collections of stars with unique properties including a glow exceeding the light its stars produced on their own and repeated Johannes Hevelius s view that the bright spots were massive and flattened due to their rotation 39 In 1750 Thomas Wright correctly speculated that the Milky Way was a flattened disk of stars and that some of the nebulae visible in the night sky might be separate Milky Ways 30 40 nbsp Photograph of the Great Andromeda Nebula by Isaac Roberts 1899 later identified as the Andromeda GalaxyToward the end of the 18th century Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance Subsequently William Herschel assembled a catalog of 5 000 nebulae 30 In 1845 Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae 41 In 1912 Vesto M Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition Slipher discovered that the spiral nebulae have high Doppler shifts indicating that they are moving at a rate exceeding the velocity of the stars he had measured He found that the majority of these nebulae are moving away from us 42 43 In 1917 Heber Doust Curtis observed nova S Andromedae within the Great Andromeda Nebula as the Andromeda Galaxy Messier object M31 was then known Searching the photographic record he found 11 more novae Curtis noticed that these novae were on average 10 magnitudes fainter than those that occurred within this galaxy As a result he was able to come up with a distance estimate of 150 000 parsecs He became a proponent of the so called island universes hypothesis which holds that spiral nebulae are actually independent galaxies 44 In 1920 a debate took place between Harlow Shapley and Heber Curtis the Great Debate concerning the nature of the Milky Way spiral nebulae and the dimensions of the universe To support his claim that the Great Andromeda Nebula is an external galaxy Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way as well as the significant Doppler shift 45 In 1922 the Estonian astronomer Ernst Opik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra galactic object 46 Using the new 100 inch Mt Wilson telescope Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables thus allowing him to estimate the distance to the nebulae they were far too distant to be part of the Milky Way 47 In 1926 Hubble produced a classification of galactic morphology that is used to this day 48 49 Modern research nbsp Rotation curve of spiral galaxy Messier 33 yellow and blue points with error bars and a predicted one from distribution of the visible matter gray line The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy 50 In 1944 Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas 51 and in 1951 it was observed This radiation is not affected by dust absorption and so its Doppler shift can be used to map the motion of the gas in this galaxy These observations led to the hypothesis of a rotating bar structure in the center of this galaxy 52 With improved radio telescopes hydrogen gas could also be traced in other galaxies In the 1970s Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas Today the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter 53 54 Beginning in the 1990s the Hubble Space Telescope yielded improved observations Among other things its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars 55 The Hubble Deep Field an extremely long exposure of a relatively empty part of the sky provided evidence that there are about 125 billion 1 25 1011 galaxies in the observable universe 56 Improved technology in detecting the spectra invisible to humans radio telescopes infrared cameras and x ray telescopes allows detection of other galaxies that are not detected by Hubble Particularly surveys in the Zone of Avoidance the region of sky blocked at visible light wavelengths by the Milky Way have revealed a number of new galaxies 57 A 2016 study published in The Astrophysical Journal led by Christopher Conselice of the University of Nottingham used 20 years of Hubble images to estimate that the observable universe contained at least two trillion 2 1012 galaxies 58 59 However later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion 2 1011 60 61 Types and morphologySee also Galactic bulge Central massive object Active galactic nucleus and Galaxy morphological classification nbsp Types of galaxies according to the Hubble classification scheme an E indicates a type of elliptical galaxy an S is a spiral and SB is a barred spiral galaxyGalaxies come in three main types ellipticals spirals and irregulars A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence Since the Hubble sequence is entirely based upon visual morphological type shape it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies 6 Many galaxies are thought to contain a supermassive black hole at their center This includes the Milky Way whose core region is called the Galactic Center 62 Ellipticals Main article Elliptical galaxy The Hubble classification system rates elliptical galaxies on the basis of their ellipticity ranging from E0 being nearly spherical up to E7 which is highly elongated These galaxies have an ellipsoidal profile giving them an elliptical appearance regardless of the viewing angle Their appearance shows little structure and they typically have relatively little interstellar matter Consequently these galaxies also have a low portion of open clusters and a reduced rate of new star formation Instead they are dominated by generally older more evolved stars that are orbiting the common center of gravity in random directions The stars contain low abundances of heavy elements because star formation ceases after the initial burst In this sense they have some similarity to the much smaller globular clusters 63 Type cD galaxies nbsp The galaxy cluster Abell 1413 is dominated by this cD elliptical galaxy designated Abell 1413 BCG It has an isophotal diameter of over 800 000 light years across Note the gravitational lensing The largest galaxies are the type cD galaxies First described in 1964 by a paper by Thomas A Matthews and others 64 they are a subtype of the more general class of D galaxies which are giant elliptical galaxies except that they are much larger They are popularly known as the supergiant elliptical galaxies and constitute the largest and most luminous galaxies known These galaxies feature a central elliptical nucleus with an extensive faint halo of stars extending to megaparsec scales 65 The profile of their surface brightnesses as a function of their radius or distance from their cores falls off more slowly than their smaller counterparts 66 The formation of these cD galaxies remains an active area of research but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters or even those outside of clusters with random overdensities 67 These processes are the mechanisms that drive the formation of fossil groups or fossil clusters where a large relatively isolated supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X rays as the residue of these galactic collisions Another older model posits the phenomenon of cooling flow where the heated gases in clusters collapses towards their centers as they cool forming stars in the process 68 a phenomenon observed in clusters such as Perseus 69 and more recently in the Phoenix Cluster 70 Shell galaxy nbsp NGC 3923 Elliptical Shell Galaxy Hubble photograph A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells About one tenth of elliptical galaxies have a shell like structure which has never been observed in spiral galaxies These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy that as the two galaxy centers approach they start to oscillate around a center point and the oscillation creates gravitational ripples forming the shells of stars similar to ripples spreading on water For example galaxy NGC 3923 has over 20 shells 71 Spirals Main articles Spiral galaxy and Barred spiral galaxy nbsp The Pinwheel Galaxy NGC 5457Spiral galaxies resemble spiraling pinwheels Though the stars and other visible material contained in such a galaxy lie mostly on a plane the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component as demonstrated by the universal rotation curve concept 72 Spiral galaxies consist of a rotating disk of stars and interstellar medium along with a central bulge of generally older stars Extending outward from the bulge are relatively bright arms In the Hubble classification scheme spiral galaxies are listed as type S followed by a letter a b or c which indicates the degree of tightness of the spiral arms and the size of the central bulge An Sa galaxy has tightly wound poorly defined arms and possesses a relatively large core region At the other extreme an Sc galaxy has open well defined arms and a small core region 73 A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy in contrast to the grand design spiral galaxy that has prominent and well defined spiral arms 74 The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges while others are thin and dense 75 76 nbsp NGC 1300 an example of a barred spiral galaxyIn spiral galaxies the spiral arms do have the shape of approximate logarithmic spirals a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars Like the stars the spiral arms rotate around the center but they do so with constant angular velocity The spiral arms are thought to be areas of high density matter or density waves 77 As stars move through an arm the space velocity of each stellar system is modified by the gravitational force of the higher density The velocity returns to normal after the stars depart on the other side of the arm This effect is akin to a wave of slowdowns moving along a highway full of moving cars The arms are visible because the high density facilitates star formation and therefore they harbor many bright and young stars 78 nbsp Hoag s Object an example of a ring galaxyBarred spiral galaxy A majority of spiral galaxies including the Milky Way galaxy have a linear bar shaped band of stars that extends outward to either side of the core then merges into the spiral arm structure 79 In the Hubble classification scheme these are designated by an SB followed by a lower case letter a b or c which indicates the form of the spiral arms in the same manner as the categorization of normal spiral galaxies Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core or else due to a tidal interaction with another galaxy 80 Many barred spiral galaxies are active possibly as a result of gas being channeled into the core along the arms 81 Our own galaxy the Milky Way is a large disk shaped barred spiral galaxy 82 about 30 kiloparsecs in diameter and a kiloparsec thick It contains about two hundred billion 2 1011 83 stars and has a total mass of about six hundred billion 6 1011 times the mass of the Sun 84 Super luminous spiral Recently researchers described galaxies called super luminous spirals They are very large with an upward diameter of 437 000 light years compared to the Milky Way s 87 400 light year diameter With a mass of 340 billion solar masses they generate a significant amount of ultraviolet and mid infrared light They are thought to have an increased star formation rate around 30 times faster than the Milky Way 85 86 Other morphologies Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies A ring galaxy has a ring like structure of stars and interstellar medium surrounding a bare core A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy 87 Such an event may have affected the Andromeda Galaxy as it displays a multi ring like structure when viewed in infrared radiation 88 A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies These are categorized as Hubble type S0 and they possess ill defined spiral arms with an elliptical halo of stars 89 barred lenticular galaxies receive Hubble classification SB0 Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral morphology An Irr I galaxy has some structure but does not align cleanly with the Hubble classification scheme Irr II galaxies do not possess any structure that resembles a Hubble classification and may have been disrupted 90 Nearby examples of dwarf irregular galaxies include the Magellanic Clouds 91 A dark or ultra diffuse galaxy is an extremely low luminosity galaxy It may be the same size as the Milky Way but have a visible star count only one percent of the Milky Way s Multiple mechanisms for producing this type of galaxy have been proposed and it is possible that different dark galaxies formed by different means 92 One candidate explanation for the low luminosity is that the galaxy lost its star forming gas at an early stage resulting in old stellar populations 93 94 Dwarfs Main article Dwarf galaxy Despite the prominence of large elliptical and spiral galaxies most galaxies are dwarf galaxies 95 They are relatively small when compared with other galactic formations being about one hundredth the size of the Milky Way with only a few billion stars Ultra compact dwarf galaxies have recently been discovered that are only 100 parsecs across 96 Many dwarf galaxies may orbit a single larger galaxy the Milky Way has at least a dozen such satellites with an estimated 300 500 yet to be discovered 97 Most of the information we have about dwarf galaxies come from observations of the local group containing two spiral galaxies the Milky Way and Andromeda and many dwarf galaxies These dwarf galaxies are classified as either irregular or dwarf elliptical dwarf spheroidal galaxies 95 A study of 27 Milky Way neighbors found that in all dwarf galaxies the central mass is approximately 10 million solar masses regardless of whether it has thousands or millions of stars This suggests that galaxies are largely formed by dark matter and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale 98 VariantsInteracting Main article Interacting galaxy nbsp The Antennae Galaxies are undergoing a collision that will result in their eventual merger Interactions between galaxies are relatively frequent and they can play an important role in galactic evolution Near misses between galaxies result in warping distortions due to tidal interactions and may cause some exchange of gas and dust 99 100 Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge The stars of interacting galaxies usually do not collide but the gas and dust within the two forms interacts sometimes triggering star formation A collision can severely distort the galaxies shapes forming bars rings or tail like structures 99 100 At the extreme of interactions are galactic mergers where the galaxies relative momentums are insufficient to allow them to pass through each other Instead they gradually merge to form a single larger galaxy Mergers can result in significant changes to the galaxies original morphology If one of the galaxies is much more massive than the other the result is known as cannibalism where the more massive larger galaxy remains relatively undisturbed and the smaller one is torn apart The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy 99 100 Starburst Main article Starburst galaxy nbsp M82 a starburst galaxy that has ten times the star formation of a normal galaxy 101 Stars are created within galaxies from a reserve of cold gas that forms giant molecular clouds Some galaxies have been observed to form stars at an exceptional rate which is known as a starburst If they continue to do so they would consume their reserve of gas in a time span less than the galaxy s lifespan Hence starburst activity usually lasts only about ten million years a relatively brief period in a galaxy s history Starburst galaxies were more common during the universe s early history 102 but still contribute an estimated 15 to total star production 103 Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars including massive stars that ionize the surrounding clouds to create H II regions 104 These stars produce supernova explosions creating expanding remnants that interact powerfully with the surrounding gas These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region Only when the available gas is nearly consumed or dispersed does the activity end 102 Starbursts are often associated with merging or interacting galaxies The prototype example of such a starburst forming interaction is M82 which experienced a close encounter with the larger M81 Irregular galaxies often exhibit spaced knots of starburst activity 105 Radio galaxy Main article Radio galaxy nbsp Hercules A supergiant elliptical radio galaxyA radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure These energetic radio lobes are powered by jets from its active galactic nucleus 106 Radio galaxies are classified according to their Fanaroff Riley classification The FR I class have lower radio luminosity and exhibit structures which are more elongated the FR II class are higher radio luminosity The correlation of radio luminosity and structure suggests that the sources in these two types of galaxies may differ 107 Radio galaxies can also be classified as giant radio galaxies GRGs whose radio emissions can extend to scales of megaparsecs 3 26 million light years Alcyoneus is an FR II class low excitation radio galaxy which has the largest observed radio emission with lobed structures spanning 5 megaparsecs 16 106 ly For comparison another similarly sized giant radio galaxy is 3C 236 with lobes 15 million light years across It should however be noted that radio emissions are not always considered part of the main galaxy itself 108 A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy s supermassive black hole Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales reaching upwards to several megaparsecs across far larger than the diameters of their host galaxies 109 A normal radio galaxy do not have a source that is a supermassive black hole or monster neutron star instead the source is synchrotron radiation from relativistic electrons accelerated by supernova These sources are comparatively short lived making the radio spectrum from normal radio galaxies an especially good way to study star formation 110 Active galaxy Main article Active galactic nucleus nbsp A jet of particles is being emitted from the core of the elliptical radio galaxy M87 Some observable galaxies are classified as active if they contain an active galactic nucleus AGN 111 A significant portion of the galaxy s total energy output is emitted by the active nucleus instead of its stars dust and interstellar medium There are multiple classification and naming schemes for AGNs but those in the lower ranges of luminosity are called Seyfert galaxies while those with luminosities much greater than that of the host galaxy are known as quasi stellar objects or quasars Models of AGNs suggest that a significant fraction of their light is shifted to far infrared frequencies because optical and UV emission in the nucleus is absorbed and remitted by dust and gas surrounding it 112 The standard model for an active galactic nucleus is based on an accretion disc that forms around a supermassive black hole SMBH at the galaxy s core region The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc 113 114 The AGN s luminosity depends on the SMBH s mass and the rate at which matter falls onto it In about 10 of these galaxies a diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light The mechanism for producing these jets is not well understood 115 Seyfert galaxy Main article Seyfert galaxy Seyfert galaxies are one of the two largest groups of active galaxies along with quasars They have quasar like nuclei very luminous distant and bright sources of electromagnetic radiation with very high surface brightnesses but unlike quasars their host galaxies are clearly detectable 116 Seen through a telescope a Seyfert galaxy appears like an ordinary galaxy with a bright star superimposed atop the core Seyfert galaxies are divided into two principal subtypes based on the frequencies observed in their spectra 117 Quasar Main article Quasar Quasars are the most energetic and distant members of active galactic nuclei Extremely luminous they were first identified as high redshift sources of electromagnetic energy including radio waves and visible light that appeared more similar to stars than to extended sources similar to galaxies Their luminosity can be 100 times that of the Milky Way 118 The nearest known quasar Markarian 231 is about 581 million light years from Earth 119 while others have been discovered as far away as UHZ1 roughly 13 2 billion light years distant 120 121 Quasars are noteworthy for providing the first demonstration of the phenomenon that gravity can act as a lens for light 122 Other AGNs Blazars are believed to be active galaxies with a relativistic jet pointed in the direction of Earth A radio galaxy emits radio frequencies from relativistic jets A unified model of these types of active galaxies explains their differences based on the observer s position 115 Possibly related to active galactic nuclei as well as starburst regions are low ionization nuclear emission line regions LINERs The emission from LINER type galaxies is dominated by weakly ionized elements The excitation sources for the weakly ionized lines include post AGB stars AGN and shocks 123 Approximately one third of nearby galaxies are classified as containing LINER nuclei 114 123 124 Luminous infrared galaxy Main article Luminous infrared galaxy Luminous infrared galaxies LIRGs are galaxies with luminosities the measurement of electromagnetic power output above 1011 L solar luminosities In most cases most of their energy comes from large numbers of young stars which heat surrounding dust which reradiates the energy in the infrared Luminosity high enough to be a LIRG requires a star formation rate of at least 18 M yr 1 Ultra luminous infrared galaxies ULIRGs are at least ten times more luminous still and form stars at rates gt 180 M yr 1 Many LIRGs also emit radiation from an AGN 125 126 Infrared galaxies emit more energy in the infrared than all other wavelengths combined with peak emission typically at wavelengths of 60 to 100 microns LIRGs are believed to be created from the strong interaction and merger of molecular gas rich spiral galaxies 127 5 1 While uncommon in the local universe LIRGs and ULIRGS were more prevalent when the universe was younger 126 Physical diametersGalaxies do not have a definite boundary by their nature and are characterized by a gradually decreasing stellar density as a function of increasing distance from their center making measurements of their true extents difficult Nevertheless astronomers over the past few decades have made several criteria in defining the sizes of galaxies Angular diameter As early as the time of Edwin Hubble in 1936 there have been attempts to characterize the diameters of galaxies The earliest efforts were based on the observed angle subtended by the galaxy and its estimated distance leading to an angular diameter also called metric diameter 128 43 This type of measurement is subject to two significant issues 1 the estimated distance to the galaxy must be corrected for the redshift related space expansion and 2 collections of angular diameter data are subject to selection bias as more distant observations preferentially select the most luminous objects 128 73 Isophotal diameter The isophotal diameter is introduced as a conventional way of measuring a galaxy s size based on its apparent surface brightness 129 Isophotes are curves in a diagram such as a picture of a galaxy that adjoins points of equal brightnesses and are useful in defining the extent of the galaxy The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond mag arcsec2 sometimes expressed as mag arcsec 2 which defines the brightness depth of the isophote To illustrate how this unit works a typical galaxy has a brightness flux of 18 mag arcsec2 at its central region This brightness is equivalent to the light of an 18th magnitude hypothetical point object like a star being spread out evenly in a one square arcsecond area of the sky 130 The isophotal diameter is typically defined as the region enclosing all the light down to 25 mag arcsec2 in the blue B band 131 which is then referred to as the D25 standard 132 Examples of isophotal diameters 25 0 B mag arcsec2 isophote galaxy diameter referenceLarge Magellanic Cloud 9 96 kiloparsecs 32 500 light years 133 Milky Way 26 8 kiloparsecs 87 400 light years 134 Messier 87 40 55 kiloparsecs 132 000 light years 135 Andromeda Galaxy 46 58 kiloparsecs 152 000 light years 136 Effective radius half light and its variations The half light radius also known as effective radius Re is a measure that is based on the galaxy s overall brightness flux This is the radius upon which half or 50 of the total brightness flux of the galaxy was emitted This was first proposed by Gerard de Vaucouleurs in 1948 137 The choice of using 50 was arbitrary but proved to be useful in further works by R A Fish in 1963 138 where he established a luminosity concentration law that relates the brightnesses of elliptical galaxies and their respective Re and by J L Sersic in 1968 139 that defined a mass radius relation in galaxies 129 In defining Re it is necessary that the overall brightness flux galaxy should be captured with a method employed by Bershady in 2000 suggesting to measure twice the size where the brightness flux of an arbitrarily chosen radius defined as the local flux divided by the overall average flux equals to 0 2 140 Using half light radius allows a rough estimate of a galaxy s size but is not particularly helpful in determining its morphology 141 Variations of this method exist In particular in the ESO Uppsala Catalogue of Galaxies values of 50 70 and 90 of the total blue light the light detected through a B band specific filter had been used to calculate a galaxy s diameter 142 Petrosian magnitude First described by V Petrosian in 1976 143 a modified version of this method has been used by the Sloan Digital Sky Survey SDSS This method employs a mathematical model on a galaxy whose radius is determined by the azimuthally horizontal averaged profile of its brightness flux In particular the SDSS employed the Petrosian magnitude in the R band 658 nm in the red part of the visible spectrum to ensure that the brightness flux of a galaxy would be captured as much as possible while counteracting the effects of background noise For a galaxy whose brightness profile is exponential it is expected to capture all of its brightness flux and 80 for galaxies that follow a profile that follows de Vaucouleurs s law 144 Petrosian magnitudes have the advantage that it is redshift and distance independent allowing the measurement of the galaxy s apparent size since the Petrosian radius is defined in terms of the galaxy s overall luminous flux 145 A critique of an earlier version of this method has been issued by IPAC 146 with the method causing a magnitude of error upwards to 10 of the values than using isophotal diameter The use of Petrosian magnitudes also have the disadvantage of missing most of the light outside the Petrosian aperture which is defined relative to the galaxy s overall brightness profile especially for elliptical galaxies with higher signal to noise ratios on higher distances and redshifts 147 A correction for this method has been issued by Graham et al in 2005 based on the assumption that galaxies follow Sersic s law 145 Near infrared method This method has been used by 2MASS as an adaptation from the previously used methods of isophotal measurement Since 2MASS operates in the near infrared which has the advantage of being able to recognize dimmer cooler and older stars it has a different form of approach compared to other methods that normally use B filter The detail of the method used by 2MASS has been described thoroughly in a document by Jarrett et al with the survey measuring several parameters 148 The standard aperture ellipse area of detection is defined by the infrared isophote at the Ks band roughly 2 2 mm wavelength of 20 mag arcsec2 Gathering the overall luminous flux of the galaxy has been employed by at least four methods the first being a circular aperture extending 7 arcseconds from the center an isophote at 20 mag arcsec2 a total aperture defined by the radial light distribution that covers the supposed extent of the galaxy and the Kron aperture defined as 2 5 times the first moment radius an integration of the flux of the total aperture 148 Magnetic fieldsGalaxies have magnetic fields of their own A galaxy s magnetic field influences its dynamics in multiple ways including affecting the formation of spiral arms and transporting angular momentum in gas clouds The latter effect is particularly important as it is a necessary factor for the gravitational collapse of those clouds and thus for star formation 149 The typical average equipartition strength for spiral galaxies is about 10 mG microgauss or 1 nT nanotesla By comparison the Earth s magnetic field has an average strength of about 0 3 G Gauss or 30 mT microtesla Radio faint galaxies like M 31 and M33 the Milky Way s neighbors have weaker fields about 5 mG while gas rich galaxies with high star formation rates like M 51 M 83 and NGC 6946 have 15 mG on average In prominent spiral arms the field strength can be up to 25 mG in regions where cold gas and dust are also concentrated The strongest total equipartition fields 50 100 mG were found in starburst galaxies for example in M 82 and the Antennae and in nuclear starburst regions such as the centers of NGC 1097 and other barred galaxies 149 Formation and evolutionMain article Galaxy formation and evolution Formation nbsp Artist s impression of a protocluster forming in the early universe 150 Current models of the formation of galaxies in the early universe are based on the LCDM model About 300 000 years after the big bang atoms of hydrogen and helium began to form in an event called recombination Nearly all the hydrogen was neutral non ionized and readily absorbed light and no stars had yet formed As a result this period has been called the dark ages It was from density fluctuations or anisotropic irregularities in this primordial matter that larger structures began to appear As a result masses of baryonic matter started to condense within cold dark matter halos 151 152 These primordial structures eventually became the galaxies we see today 153 nbsp Artist s impression of a young galaxy accreting materialEarly galaxy formation Evidence for the appearance of galaxies very early in the Universe s history was found in 2006 when it was discovered that the galaxy IOK 1 has an unusually high redshift of 6 96 corresponding to just 750 million years after the Big Bang and making it the most distant and earliest to form galaxy seen at that time 154 While some scientists have claimed other objects such as Abell 1835 IR1916 have higher redshifts and therefore are seen in an earlier stage of the universe s evolution IOK 1 s age and composition have been more reliably established In December 2012 astronomers reported that UDFj 39546284 is the most distant object known and has a redshift value of 11 9 The object estimated to have existed around 380 million years 155 after the Big Bang which was about 13 8 billion years ago 156 is about 13 42 billion light travel distance years away The existence of galaxies so soon after the Big Bang suggests that protogalaxies must have grown in the so called dark ages 151 As of May 5 2015 the galaxy EGS zs8 1 is the most distant and earliest galaxy measured forming 670 million years after the Big Bang The light from EGS zs8 1 has taken 13 billion years to reach Earth and is now 30 billion light years away because of the expansion of the universe during 13 billion years 157 158 159 160 161 On 17 August 2022 NASA released a large mosaic image of 690 individual frames taken by the Near Infrared Camera NIRCam on the James Webb Space Telescope JWST of numerous very early galaxies 162 163 164 In May 2023 a study in the journal Nature identified an ultra faint galaxy named JD1 165 Galaxy JD1 was observed by the JWST using the near infrared spectrograph instrument NIRSpec and was found to have a distance value of redshift z 9 79 165 This means that JD1 was observed at 480 million years after the Big Bang when the universe was only about 4 of its present age 165 166 Observations of this ultra faint galaxy were aided by the effect of a gravitational lens in the galaxy cluster Abell 2744 which helped make the image of JD1 larger and 13 times brighter than it otherwise would be 165 166 This effect and the use of the JWST s NIRCam showed JD1 s structure to be three starforming clumps of dust and gas 166 nbsp Different components of near infrared background light detected by the Hubble Space Telescope in deep sky surveys 167 The detailed process by which the earliest galaxies formed is an open question in astrophysics Theories can be divided into two categories top down and bottom up In top down correlations such as the Eggen Lynden Bell Sandage ELS model protogalaxies form in a large scale simultaneous collapse lasting about one hundred million years 168 In bottom up theories such as the Searle Zinn SZ model small structures such as globular clusters form first and then a number of such bodies accrete to form a larger galaxy 169 Once protogalaxies began to form and contract the first halo stars called Population III stars appeared within them These were composed almost entirely of hydrogen and helium and may have been more massive than 100 times the Sun s mass If so these huge stars would have quickly consumed their supply of fuel and became supernovae releasing heavy elements into the interstellar medium 170 This first generation of stars re ionized the surrounding neutral hydrogen creating expanding bubbles of space through which light could readily travel 171 In June 2015 astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z 6 60 Such stars are likely to have existed in the very early universe i e at high redshift and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it 172 173 In late 2023 scientists reported results suggesting that newborn galaxies in the very early universe were banana shaped much to the surprise of researchers 174 175 Evolution Within a billion years of a galaxy s formation key structures begin to appear 176 Globular clusters the central supermassive black hole and a galactic bulge of metal poor Population II stars form The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added 177 During this early epoch galaxies undergo a major burst of star formation 178 During the following two billion years the accumulated matter settles into a galactic disc 179 A galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life 180 This matter is mostly hydrogen and helium The cycle of stellar birth and death slowly increases the abundance of heavy elements eventually allowing the formation of planets 181 Hubble eXtreme Deep Field XDF nbsp XDF view field compared to the angular size of the Moon Several thousand galaxies each consisting of billions of stars are in this small view nbsp XDF 2012 view Each light speck is a galaxy some of which are as old as 13 2 billion years 182 the observable universe is estimated to contain 200 billion to two trillion galaxies nbsp XDF image shows from left fully mature galaxies nearly mature galaxies from five to nine billion years ago and protogalaxies blazing with young stars beyond nine billion years Star formation rates in galaxies depend upon their local environment Isolated void galaxies have highest rate per stellar mass with field galaxies associated with spiral galaxies having lower rates and galaxies in dense cluster having the lowest rates 183 The evolution of galaxies can be significantly affected by interactions and collisions Mergers of galaxies were common during the early epoch and the majority of galaxies were peculiar in morphology 184 Given the distances between the stars the great majority of stellar systems in colliding galaxies will be unaffected However gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails Examples of these formations can be seen in NGC 4676 185 or the Antennae Galaxies 186 The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km s and depending upon the lateral movements the two might collide in about five to six billion years Although the Milky Way has never collided with a galaxy as large as Andromeda before it has collided and merged with other galaxies in the past 187 Cosmological simulations indicate that 11 billion years ago it merged with a particularly large galaxy that has been labeled the Kraken 188 189 Such large scale interactions are rare As time passes mergers of two systems of equal size become less common Most bright galaxies have remained fundamentally unchanged for the last few billion years and the net rate of star formation probably also peaked about ten billion years ago 190 Future trends Main article Future of an expanding universe Spiral galaxies like the Milky Way produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms 191 Elliptical galaxies are largely devoid of this gas and so form few new stars 192 The supply of star forming material is finite once stars have converted the available supply of hydrogen into heavier elements new star formation will come to an end 193 194 The current era of star formation is expected to continue for up to one hundred billion years and then the stellar age will wind down after about ten trillion to one hundred trillion years 1013 1014 years as the smallest longest lived stars in the visible universe tiny red dwarfs begin to fade At the end of the stellar age galaxies will be composed of compact objects brown dwarfs white dwarfs that are cooling or cold black dwarfs neutron stars and black holes Eventually as a result of gravitational relaxation all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions 193 195 Larger scale structuresMain articles Observable universe Large scale structure Galaxy filament and Galaxy groups and clusters nbsp Seyfert s Sextet is an example of a compact galaxy group Deep sky surveys show that galaxies are often found in groups and clusters Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past billion years are relatively scarce Only about 5 of the galaxies surveyed are truly isolated however they may have interacted and even merged with other galaxies in the past and may still be orbited by smaller satellite galaxies On the largest scale the universe is continually expanding resulting in an average increase in the separation between individual galaxies see Hubble s law Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction These associations formed early as clumps of dark matter pulled their respective galaxies together Nearby groups later merged to form larger scale clusters This ongoing merging process as well as an influx of infalling gas heats the intergalactic gas in a cluster to very high temperatures of 30 100 megakelvins 196 About 70 80 of a cluster s mass is in the form of dark matter with 10 30 consisting of this heated gas and the remaining few percent in the form of galaxies 197 Most galaxies are gravitationally bound to a number of other galaxies These form a fractal like hierarchical distribution of clustered structures with the smallest such associations being termed groups A group of galaxies is the most common type of galactic cluster these formations contain the majority of galaxies as well as most of the baryonic mass in the universe 198 199 To remain gravitationally bound to such a group each member galaxy must have a sufficiently low velocity to prevent it from escaping see Virial theorem If there is insufficient kinetic energy however the group may evolve into a smaller number of galaxies through mergers 200 Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity 201 Clusters of galaxies are often dominated by a single giant elliptical galaxy known as the brightest cluster galaxy which over time tidally destroys its satellite galaxies and adds their mass to its own 202 nbsp Southern plane of the Milky Way from submillimeter wavelengths 203 Superclusters contain tens of thousands of galaxies which are found in clusters groups and sometimes individually At the supercluster scale galaxies are arranged into sheets and filaments surrounding vast empty voids 204 Above this scale the universe appears to be the same in all directions isotropic and homogeneous 205 though this notion has been challenged in recent years by numerous findings of large scale structures that appear to be exceeding this scale The Hercules Corona Borealis Great Wall currently the largest structure in the universe found so far is 10 billion light years three gigaparsecs in length 206 207 208 The Milky Way galaxy is a member of an association named the Local Group a relatively small group of galaxies that has a diameter of approximately one megaparsec The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group many of the other member galaxies are dwarf companions of these two 209 The Local Group itself is a part of a cloud like structure within the Virgo Supercluster a large extended structure of groups and clusters of galaxies centered on the Virgo Cluster 210 In turn the Virgo Supercluster is a portion of the Laniakea Supercluster 211 Multi wavelength observationSee also Observational astronomy nbsp This ultraviolet image of Andromeda shows blue regions containing young massive stars Advances in astronomy have always been driven by technology After centuries of success in optical astronomy recent decades have seen major progress in other regions of the electromagnetic spectrum 212 The dust present in the interstellar medium is opaque to visual light It is more transparent to far infrared which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail 213 Infrared is also used to observe distant red shifted galaxies that were formed much earlier Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum so high altitude or space based telescopes are used for infrared astronomy 214 The first non visual study of galaxies particularly active galaxies was made using radio frequencies The Earth s atmosphere is nearly transparent to radio between 5 MHz and 30 GHz The ionosphere blocks signals below this range 215 Large radio interferometers have been used to map the active jets emitted from active nuclei Radio telescopes can also be used to observe neutral hydrogen via 21 cm radiation including potentially the non ionized matter in the early universe that later collapsed to form galaxies 216 Ultraviolet and X ray telescopes can observe highly energetic galactic phenomena Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole 217 The distribution of hot gas in galactic clusters can be mapped by X rays The existence of supermassive black holes at the cores of galaxies was confirmed through X ray astronomy 218 Gallery nbsp Galaxies left top right bottom NGC 7541 NGC 3021 NGC 5643 NGC 3254 NGC 3147 NGC 105 NGC 2608 NGC 3583 NGC 3147 MRK 1337 NGC 5861 NGC 2525 NGC 1015 UGC 9391 NGC 691 NGC 7678 NGC 2442 NGC 5468 NGC 5917 NGC 4639 NGC 3972 The Antennae Galaxies NGC 5584 M106 NGC 7250 NGC 3370 NGC 5728 NGC 4424 NGC 1559 NGC 3982 NGC 1448 NGC 4680 M101 NGC 1365 NGC 7329 NGC 3447 nbsp 19 face on spiral galaxies from the James Webb Space Telescope in near and mid infrared light Older stars appear blue here and are clustered at the galaxies cores Glowing dust showing where it exists around and between stars appearing in shades of red and orange Stars that haven t yet fully formed and are encased in gas and dust appear bright red 219 See also nbsp solar system portal nbsp outer space portal nbsp astronomy portal nbsp stars portalBright early galaxies Dark galaxy Galactic orientation Galaxy formation and evolution Illustris project List of galaxies List of nearest galaxies List of largest galaxies Low surface brightness galaxy Outline of galaxies Timeline of knowledge about galaxies clusters of galaxies and large scale structure UniverseMachineNotesReferences Sparke amp Gallagher 2000 p i Hupp E Roy S Watzke M August 12 2006 NASA Finds Direct Proof of Dark Matter NASA Archived from the original on March 28 2020 Retrieved April 17 2007 Howell Elizabeth Harvey Ailsa February 11 2022 How many stars are in the universe Can we estimate the total number of stars Space com Retrieved July 16 2022 Strigari Louis E Bullock James S Kaplinghat Manoj Simon Joshua D Geha Marla Willman Beth Walker Matthew G 2008 A common mass scale for satellite galaxies of the Milky Way Nature 454 7208 1096 1097 arXiv 0808 3772 doi 10 1038 nature07222 ISSN 0028 0836 PMID 18756252 Hoover A June 16 2003 UF Astronomers Universe Slightly Simpler Than Expected Hubble News Desk Archived from the original on July 20 2011 Retrieved March 4 2011 Based upon Graham A W Guzman R 2003 HST Photometry of Dwarf Elliptical Galaxies in Coma and an Explanation for the Alleged Structural Dichotomy between Dwarf and Bright Elliptical Galaxies The Astronomical Journal 125 6 2936 2950 arXiv astro ph 0303391 Bibcode 2003AJ 125 2936G doi 10 1086 374992 S2CID 13284968 a b Jarrett T H Near Infrared Galaxy Morphology Atlas California Institute of Technology Archived from the original on August 2 2012 Retrieved January 9 2007 Finley D Aguilar D November 2 2005 Astronomers Get Closest Look Yet At Milky Way s Mysterious Core National Radio Astronomy Observatory Archived from the original on December 20 2015 Retrieved August 10 2006 Astronomers were wrong about the number of galaxies in universe The Jerusalem Post January 14 2021 Archived from the original on January 14 2021 Retrieved January 14 2021 Saunders Toby July 25 2023 How many galaxies are in the Universe A lot more than you d think BBC Science Focus Retrieved January 9 2024 Galaxy Clusters and Large Scale Structure University of Cambridge Archived from the original on May 24 2012 Retrieved January 15 2007 Gibney Elizabeth 2014 Earth s new address Solar System Milky Way Laniakea Nature doi 10 1038 nature 2014 15819 S2CID 124323774 C T Onions et al The Oxford Dictionary of English Etymology Oxford 1966 p 385 a b Harper D galaxy Online Etymology Dictionary Archived from the original on May 27 2012 Retrieved November 11 2011 Waller amp Hodge 2003 p 91 Konecny Lubomir Emblematics Agriculture and Mythography in The Origin of the Milky Way PDF Academy of Sciences of the Czech Republic Archived from the original PDF on July 20 2006 Retrieved January 5 2007 Rao J September 2 2005 Explore the Archer s Realm Space com Archived from the original on October 31 2010 Retrieved January 3 2007 Results for object MESSIER 109 M109 NASA IPAC Extragalactic Database Retrieved January 13 2023 SkyServer About the SDSS SkyServer Archived from the original on January 12 2024 Retrieved March 3 2024 Plutarch 2006 The Complete Works Volume 3 Essays and Miscellanies Echo Library p 66 ISBN 978 1 4068 3224 2 Archived from the original on March 24 2021 Retrieved July 25 2018 a b c Montada J P September 28 2007 Ibn Bajja Stanford Encyclopedia of Philosophy Archived from the original on March 16 2020 Retrieved July 11 2008 Heidarzadeh 2008 pp 23 25 Mohamed 2000 pp 49 50 Bouali H E Zghal M Lakhdar Z B 2005 Popularisation of Optical Phenomena Establishing the First Ibn Al Haytham Workshop on Photography PDF The Education and Training in Optics and Photonics Conference Archived PDF from the original on May 24 2011 Retrieved July 8 2008 O Connor John J Robertson Edmund F Abu Arrayhan Muhammad ibn Ahmad al Biruni MacTutor History of Mathematics Archive University of St Andrews Heidarzadeh 2008 p 25 Table 2 1 Livingston J W 1971 Ibn Qayyim al Jawziyyah A Fourteenth Century Defense against Astrological Divination and Alchemical Transmutation Journal of the American Oriental Society 91 1 96 103 99 doi 10 2307 600445 JSTOR 600445 Galilei Galileo 1610 Sidereus Nuncius Venice Italy Thomas Baglioni pp 15 16 Retrieved January 10 2024 English translation Galilei Galileo 1880 The Sidereal Messenger Translated by Carlos Edward Stafford London England Rivingtons pp 42 43 Retrieved January 10 2024 O Connor J J Robertson E F November 2002 Galileo Galilei University of St Andrews Archived from the original on May 30 2012 Retrieved January 8 2007 Wright Thomas 1750 An Original Theory or New Hypothesis of the Universe London England H Chapelle pp 48 Archived November 20 2016 at the Wayback Machine the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space without order or design this phaenomenon is no other than a certain effect arising from the observer s situation To a spectator placed in an indefinite space it i e the Milky Way Via Lactea is a vast ring of stars On page 73 Archived November 20 2016 at the Wayback Machine Wright called the Milky Way the Vortex Magnus the great whirlpool and estimated its diameter at 8 64 1012 miles 13 9 1012 km a b c d Evans J C November 24 1998 Our Galaxy George Mason University Archived from the original on June 30 2012 Retrieved January 4 2007 Kant Immanuel November 20 2016 1755 Allgemeine Naturgeschichte und Theorie des Himmels Universal Natural History and Theory of the Heavens Konigsberg and Leipzig Germany Johann Friederich Petersen ISBN 978 3 89131 002 1 Archived 2016 11 20 at the Wayback Machine Available in English translation by Ian Johnston at Vancouver Island University British Columbia Canada Archived August 29 2014 at the Wayback Machine Herschel William 1785 XII On the construction of the heavens Giving Some Accounts of the Present Undertakings Studies and Labours of the Ingenious in Many Considerable Parts of the World Philosophical Transactions of the Royal Society of London Vol 75 London pp 213 266 doi 10 1098 rstl 1785 0012 ISSN 0261 0523 S2CID 186213203 Archived from the original on November 20 2016 Retrieved January 27 2016 Herschel s diagram of the galaxy appears immediately after the article s last page Paul 1993 pp 16 18 Trimble V 1999 Robert Trumpler and the Non transparency of Space Bulletin of the American Astronomical Society 31 31 1479 Bibcode 1999AAS 195 7409T a b Kepple amp Sanner 1998 p 18 a b The Large Magellanic Cloud LMC Observatoire de Paris March 11 2004 Archived from the original on June 22 2017 Abd al Rahman Al Sufi December 7 903 May 25 986 A D Observatoire de Paris Archived from the original on April 16 2007 Retrieved April 19 2007 Gordon Kurtiss J History of our Understanding of a Spiral Galaxy Messier 33 Caltech edu Archived from the original on January 25 2021 Retrieved June 11 2018 Kant Immanuel Universal Natural History and Theory of the Heavens 1755 See text quoted from Wright s An original theory or new hypothesis of the Universe in Dyson F 1979 Disturbing the Universe Pan Books p 245 ISBN 978 0 330 26324 5 Archived from the original on March 24 2021 Retrieved July 25 2018 William Rosse parsonstown info March 24 2021 Retrieved January 11 2024 Slipher V M 1913 The radial velocity of the Andromeda Nebula Lowell Observatory Bulletin 1 8 56 57 Bibcode 1913LowOB 2 56S Slipher V M 1915 Spectrographic Observations of Nebulae Popular Astronomy Vol 23 pp 21 24 Bibcode 1915PA 23 21S Curtis H D 1988 Novae in Spiral Nebulae and the Island Universe Theory Publications of the Astronomical Society of the Pacific 100 6 Bibcode 1988PASP 100 6C doi 10 1086 132128 Weaver H F Robert Julius Trumpler US National Academy of Sciences Archived from the original on December 24 2013 Retrieved January 5 2007 Opik E 1922 An estimate of the distance of the Andromeda Nebula The Astrophysical Journal 55 406 Bibcode 1922ApJ 55 406O doi 10 1086 142680 Hubble E P 1929 A spiral nebula as a stellar system Messier 31 The Astrophysical Journal 69 103 158 Bibcode 1929ApJ 69 103H doi 10 1086 143167 Hubble Edwin 1926 No 324 Extra galactic nebulae Contributions from the Mount Wilson Observatory 324 Carnegie Institution of Washington 1 49 Bibcode 1926CMWCI 324 1H Sandage A 1989 Edwin Hubble 1889 1953 Journal of the Royal Astronomical Society of Canada 83 6 351 362 Bibcode 1989JRASC 83 351S Archived from the original on May 30 2012 Retrieved January 8 2007 Corbelli E Salucci P 2000 The extended rotation curve and the dark matter halo of M33 Monthly Notices of the Royal Astronomical Society 311 2 441 447 arXiv astro ph 9909252 Bibcode 2000MNRAS 311 441C doi 10 1046 j 1365 8711 2000 03075 x Tenn J Hendrik Christoffel van de Hulst Sonoma State University Archived from the original on May 29 2012 Retrieved January 5 2007 Lopez Corredoira M et al 2001 Searching for the in plane Galactic bar and ring in DENIS Astronomy and Astrophysics 373 1 139 152 arXiv astro ph 0104307 Bibcode 2001A amp A 373 139L doi 10 1051 0004 6361 20010560 S2CID 18399375 Rubin V C 1983 Dark matter in spiral galaxies Scientific American Vol 248 no 6 pp 96 106 Bibcode 1983SciAm 248f 96R doi 10 1038 scientificamerican0683 96 Rubin V C 2000 One Hundred Years of Rotating Galaxies Publications of the Astronomical Society of the Pacific 112 772 747 750 Bibcode 2000PASP 112 747R doi 10 1086 316573 S2CID 122927800 Hubble Rules Out a Leading Explanation for Dark Matter Hubble News Desk October 17 1994 Archived from the original on August 1 2012 Retrieved January 8 2007 How many galaxies are there NASA November 27 2002 Archived from the original on July 11 2012 Retrieved January 8 2007 Kraan Korteweg R C Juraszek S 2000 Mapping the hidden Universe The galaxy distribution in the Zone of Avoidance Publications of the Astronomical Society of Australia 17 1 6 12 arXiv astro ph 9910572 Bibcode 2000PASA 17 6K doi 10 1071 AS00006 S2CID 17900483 Conselice Christopher J et al 2016 The Evolution of Galaxy Number Density at z lt 8 and its Implications The Astrophysical Journal 830 2 83 arXiv 1607 03909 Bibcode 2016ApJ 830 83C doi 10 3847 0004 637X 830 2 83 S2CID 17424588 Fountain Henry October 17 2016 Two Trillion Galaxies at the Very Least The New York Times Archived from the original on December 31 2019 Retrieved October 17 2016 Lauer Tod R et al January 11 2021 New Horizons Observations of the Cosmic Optical Background The Astrophysical Journal 906 2 77 arXiv 2011 03052 Bibcode 2021ApJ 906 77L doi 10 3847 1538 4357 abc881 hdl 1721 1 133770 ISSN 1538 4357 S2CID 226277978 New Horizons spacecraft answers the question How dark is space phys org Archived from the original on January 15 2021 Retrieved January 15 2021 Fraknoi et al 2023 pp 851 856 907 915 Barstow M A 2005 Elliptical Galaxies Leicester University Physics Department Archived from the original on July 29 2012 Retrieved June 8 2006 Matthews Thomas A Morgan William W Schmidt Maarten 1964 A Discussion of Galaxies Indentified sic with Radio Sources The Astrophysical Journal 140 35 Bibcode 1964ApJ 140 35M doi 10 1086 147890 Discoveries Highlights Tracing the Growth of Galaxies February 6 2017 Tonry John L 1987 Properties of cD Galaxies Structure and Dynamics of Elliptical Galaxies Vol 127 p 89 Bibcode 1987IAUS 127 89T doi 10 1007 978 94 009 3971 4 7 ISBN 978 90 277 2586 8 S2CID 117980521 Tovmassian Hrant M Andernach Heinz December 11 2012 On the formation of cD galaxies and their parent clusters Formation of cD galaxies and parent clusters Monthly Notices of the Royal Astronomical Society 427 3 2047 2056 arXiv 1212 0238 doi 10 1111 j 1365 2966 2012 22044 x Bender Ralf Kormendy John Cornell Mark E Fisher David B June 30 2015 Structure and Formation of cD Galaxies NGC 6166 in Abell 2199 The Astrophysical Journal 807 1 56 arXiv 1411 2598 Bibcode 2015ApJ 807 56B doi 10 1088 0004 637X 807 1 56 ISSN 1538 4357 This idea was entertained in the heyday of the cooling flow problem when we observed large amounts of X ray emitting hot gas in clusters but could not measure temperature profiles This possibility is now regarded as a non starter Fabian A C Nulsen P E J 1977 Subsonic accretion of cooling gas in clusters of galaxies Monthly Notices of the Royal Astronomical Society 180 3 479 Bibcode 1977MNRAS 180 479F doi 10 1093 mnras 180 3 479 McDonald M McNamara B R Voit G M Bayliss M Benson B A Brodwin M Canning R E A Florian M K Garmire G P Gaspari M Gladders M D Hlavacek Larrondo J Kara E Reichardt C L Russell H R November 1 2019 Anatomy of a Cooling Flow The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster The Astrophysical Journal 885 1 63 arXiv 1904 08942 Bibcode 2019ApJ 885 63M doi 10 3847 1538 4357 ab464c ISSN 0004 637X Galactic onion www spacetelescope org Archived from the original on August 6 2020 Retrieved May 11 2015 Williams M J Bureau M Cappellari M 2010 Kinematic constraints on the stellar and dark matter content of spiral and S0 galaxies Monthly Notices of the Royal Astronomical Society 400 4 1665 1689 arXiv 0909 0680 Bibcode 2009MNRAS 400 1665W doi 10 1111 j 1365 2966 2009 15582 x S2CID 17940107 Smith G March 6 2000 Galaxies The Spiral Nebulae University of California San Diego Center for Astrophysics amp Space Sciences Archived from the original on July 10 2012 Retrieved November 30 2006 Van den Bergh 1998 p 17 Obreschkow D Glazebrook K February 28 2014 Fundamental Mass Spin Morphology Relation of Spiral Galaxies The Astrophysical Journal 784 1 26 arXiv 1312 4543 Bibcode 2014ApJ 784 26O doi 10 1088 0004 637X 784 1 26 ISSN 0004 637X Fat or flat Getting galaxies into shape Archived March 24 2021 at the Wayback Machine phys org February 2014 Bertin amp Lin 1996 pp 65 85 Belkora 2003 p 355 Eskridge P B Frogel J A 1999 What is the True Fraction of Barred Spiral Galaxies Astrophysics and Space Science 269 270 427 430 Bibcode 1999Ap amp SS 269 427E doi 10 1023 A 1017025820201 S2CID 189840251 Bournaud F Combes F 2002 Gas accretion on spiral galaxies Bar formation and renewal Astronomy and Astrophysics 392 1 83 102 arXiv astro ph 0206273 Bibcode 2002A amp A 392 83B doi 10 1051 0004 6361 20020920 S2CID 17562844 Knapen J H Perez Ramirez D Laine S 2002 Circumnuclear regions in barred spiral galaxies II Relations to host galaxies Monthly Notices of the Royal Astronomical Society 337 3 808 828 arXiv astro ph 0207258 Bibcode 2002MNRAS 337 808K doi 10 1046 j 1365 8711 2002 05840 x S2CID 10845683 Alard C 2001 Another bar in the Bulge Astronomy and Astrophysics Letters 379 2 L44 L47 arXiv astro ph 0110491 Bibcode 2001A amp A 379L 44A doi 10 1051 0004 6361 20011487 S2CID 18018228 Sanders R January 9 2006 Milky Way galaxy is warped and vibrating like a drum UCBerkeley News Archived from the original on January 18 2014 Retrieved May 24 2006 Bell G R Levine S E 1997 Mass of the Milky Way and Dwarf Spheroidal Stream Membership Bulletin of the American Astronomical Society 29 2 1384 Bibcode 1997AAS 19110806B We Just Discovered a New Type of Colossal Galaxy Futurism March 21 2016 Archived from the original on March 24 2021 Retrieved March 21 2016 Ogle Patrick M Lanz Lauranne Nader Cyril Helou George January 1 2016 Superluminous Spiral Galaxies The Astrophysical Journal 817 2 109 arXiv 1511 00659 Bibcode 2016ApJ 817 109O doi 10 3847 0004 637X 817 2 109 ISSN 0004 637X S2CID 35287348 Gerber R A Lamb S A Balsara D S 1994 Ring Galaxy Evolution as a Function of Intruder Mass Bulletin of the American Astronomical Society 26 911 Bibcode 1994AAS 184 3204G ISO unveils the hidden rings of Andromeda Press release European Space Agency October 14 1998 Archived from the original on August 28 1999 Retrieved May 24 2006 Spitzer Reveals What Edwin Hubble Missed Harvard Smithsonian Center for Astrophysics May 31 2004 Archived from the original on September 7 2006 Retrieved December 6 2006 Barstow M A 2005 Irregular Galaxies University of Leicester Archived from the original on February 27 2012 Retrieved December 5 2006 Fraknoi et al 2023 pp 879 Pandya Viraj Romanowsky Aaron J Laine Seppo Brodie Jean P Johnson Benjamin D Glaccum William Villaume Alexa Cuillandre Jean Charles Gwyn Stephen Krick Jessica Lasker Ronald Martin Navarro Ignacio Martinez Delgado David van Dokkum Pieter May 1 2018 The Stellar Populations of Two Ultra diffuse Galaxies from Optical and Near infrared Photometry The Astrophysical Journal 858 1 29 arXiv 1711 05272 Bibcode 2018ApJ 858 29P doi 10 3847 1538 4357 aab498 ISSN 0004 637X Overbye Dennis January 26 2024 What Do You Call a Galaxy Without Stars To dark matter and dark energy add dark galaxies collections of stars so sparse and faint that they are all but invisible The New York Times Archived from the original on January 26 2024 van Dokkum Pieter G Abraham Roberto Merritt Allison Zhang Jielai Geha Marla Conroy Charlie January 2015 Forty Seven Milky Way Sized Extremely Diffuse Galaxies in the Coma Cluster The Astrophysical Journal Letters 798 2 L45 arXiv 1410 8141 Bibcode 2015ApJ 798L 45V doi 10 1088 2041 8205 798 2 L45 ISSN 2041 8205 a b Mateo Mario 1998 Dwarf Galaxies of the Local Group Annual Review of Astronomy and Astrophysics 36 1 435 506 arXiv astro ph 9810070 Bibcode 1998ARA amp A 36 435M doi 10 1146 annurev astro 36 1 435 ISSN 0066 4146 S2CID 119333888 Phillipps S Drinkwater M J Gregg M D Jones J B 2001 Ultracompact Dwarf Galaxies in the Fornax Cluster The Astrophysical Journal 560 1 201 206 arXiv astro ph 0106377 Bibcode 2001ApJ 560 201P doi 10 1086 322517 S2CID 18297376 Groshong K April 24 2006 Strange satellite galaxies revealed around Milky Way New Scientist Archived from the original on July 2 2015 Retrieved January 10 2007 Schirber M August 27 2008 No Slimming Down for Dwarf Galaxies ScienceNOW Archived from the original on May 30 2020 Retrieved August 27 2008 a b c Galaxy Interactions University of Maryland Department of Astronomy Archived from the original on May 9 2006 Retrieved December 19 2006 a b c Interacting Galaxies Swinburne University Archived from the original on July 7 2012 Retrieved December 19 2006 Happy Sweet Sixteen Hubble Telescope NASA April 24 2006 Archived from the original on July 14 2012 Retrieved August 10 2006 a b Starburst Galaxies Harvard Smithsonian Center for Astrophysics August 29 2006 Archived from the original on March 16 2019 Retrieved August 10 2006 Kennicutt R C Jr et al 2005 Demographics and Host Galaxies of Starbursts Starbursts From 30 Doradus to Lyman Break Galaxies Springer p 187 Bibcode 2005ASSL 329 187K doi 10 1007 1 4020 3539 X 33 ISBN 978 1 4020 3538 8 Smith G July 13 2006 Starbursts amp Colliding Galaxies University of California San Diego Center for Astrophysics amp Space Sciences Archived from the original on July 7 2012 Retrieved August 10 2006 Keel B September 2006 Starburst Galaxies University of Alabama Archived from the original on June 4 2012 Retrieved December 11 2006 Adams David J Adams David John Cayless Alan Jones Anthony W 2004 Jones Mark H Adams David J Lambourne Robert J eds An Introduction to Galaxies and Cosmology Cambridge University Press pp 142 144 ISBN 978 0 521 54623 2 Kembhavi Ajit K Narlikar Jayant V 2012 1999 RADIO PROPERTIES Quasars and Active Galactic Nuclei An Introduction Cambridge Cambridge University Press pp 214 272 ISBN 978 1 139 17440 4 Introductory Astronomy Double Lobed Radio Galaxies Department of Astronomy University of Maryland Archived from the original on September 1 2022 Retrieved September 1 2022 Oei Martijn S S L van Weeren Reinout J Hardcastle Martin J Botteon Andrea Shimwell Tim W Dabhade Pratik Gast Aivin R D J G I B Rottgering Huub J A Bruggen Marcus Tasse Cyril Williams Wendy L Shulevski Aleksandar April 1 2022 The discovery of a radio galaxy of at least 5 Mpc Astronomy amp Astrophysics 660 A2 arXiv 2202 05427 Bibcode 2022A amp A 660A 2O doi 10 1051 0004 6361 202142778 ISSN 0004 6361 Condon J J September 1992 Radio Emission from Normal Galaxies Annual Review of Astronomy and Astrophysics 30 1 575 611 Bibcode 1992ARA amp A 30 575C doi 10 1146 annurev aa 30 090192 003043 ISSN 0066 4146 Fraknoi et al 2023 p 906 Zakamska Nadia L Strauss Michael A Krolik Julian H Collinge Matthew J Hall Patrick B Hao Lei Heckman Timothy M Ivezi eljko Richards Gordon T Schlegel David J Schneider Donald P Strateva Iskra Vanden Berk Daniel E Anderson Scott F Brinkmann Jon November 2003 Candidate Type II Quasars from the Sloan Digital Sky Survey I Selection and Optical Properties of a Sample at 0 3 lt Z The Astronomical Journal 126 5 2125 2144 arXiv astro ph 0309551 doi 10 1086 378610 ISSN 0004 6256 Fraknoi et al 2023 pp 907 915 a b Keel W C 2000 Introducing Active Galactic Nuclei University of Alabama Archived from the original on July 27 2012 Retrieved December 6 2006 a b Lochner J Gibb M A Monster in the Middle NASA Archived from the original on July 10 2012 Retrieved December 20 2006 Peterson Bradley M 1997 An Introduction to Active Galactic Nuclei Cambridge University Press ISBN 978 0 521 47911 0 Peterson Bradley M August 1 2008 The central black hole and relationships with the host galaxy PDF New Astronomy Reviews Active Galactic Nuclei at the Highest Angular Resolution Theory and Observations 52 6 240 252 Bibcode 2008NewAR 52 240P doi 10 1016 j newar 2008 06 005 ISSN 1387 6473 S2CID 121460317 Fraknoi et al 2023 pp 899 906 Kovacevic Andjelka B Yi Tignfeng Dai Xinyu Yang Xing Cvorovic Hajdinjak Iva Popovic Luka C May 21 2020 Confirmed short periodic variability of subparsec supermassive binary black hole candidate Mrk 231 Monthly Notices of the Royal Astronomical Society 494 3 4069 4076 arXiv 2003 06359 doi 10 1093 mnras staa737 ISSN 0035 8711 UHZ1 Distant Galaxy and Black Hole Astronomy Picture of the Day November 10 2023 Retrieved February 18 2024 Bogdan Akos Goulding Andy D Natarajan Priyamvada Kovacs Orsolya E Tremblay Grant R Chadayammuri Urmila Volonteri Marta Kraft Ralph P Forman William R Jones Christine Churazov Eugene Zhuravleva Irina January 2024 Evidence for heavy seed origin of early supermassive black holes from a z 10 X ray quasar Nature Astronomy 8 1 126 133 arXiv 2305 15458 Bibcode 2024NatAs 8 126B doi 10 1038 s41550 023 02111 9 ISSN 2397 3366 S2CID 258887541 Blandford R D Narayan R September 1992 Cosmological Applications of Gravitational Lensing Annual Review of Astronomy and Astrophysics 30 1 311 358 doi 10 1146 annurev aa 30 090192 001523 ISSN 0066 4146 a b Heckman T M 1980 An optical and radio survey of the nuclei of bright galaxies Activity in normal galactic nuclei Astronomy and Astrophysics 87 152 164 Bibcode 1980A amp A 87 152H Ho L C Filippenko A V Sargent W L W 1997 A Search for Dwarf Seyfert Nuclei V Demographics of Nuclear Activity in Nearby Galaxies The Astrophysical Journal 487 2 568 578 arXiv astro ph 9704108 Bibcode 1997ApJ 487 568H doi 10 1086 304638 S2CID 16742031 Perez Torres Miguel Mattila Seppo Alonso Herrero Almudena Aalto Susanne Efstathiou Andreas December 2021 Star formation and nuclear activity in luminous infrared galaxies an infrared through radio review The Astronomy and Astrophysics Review 29 1 2 arXiv 2010 05072 Bibcode 2021A amp ARv 29 2P doi 10 1007 s00159 020 00128 x ISSN 0935 4956 S2CID 253687419 a b Bellocchi E Pereira Santaella M Colina L Labiano A Sanchez Garcia M Alonso Herrero A Arribas S Garcia Burillo S Villar Martin M Rigopoulou D Valentino F Puglisi A Diaz Santos T Cazzoli S Usero A August 2022 Compact molecular gas emission in local LIRGs among low and high z galaxies Astronomy amp Astrophysics 664 A60 arXiv 2204 02055 Bibcode 2022A amp A 664A 60B doi 10 1051 0004 6361 202142802 ISSN 0004 6361 Sanders D B Mirabel I F September 1996 Luminous Infrared Galaxies PDF Annual Review of Astronomy and Astrophysics 34 1 749 792 Bibcode 1996ARA amp A 34 749S doi 10 1146 annurev astro 34 1 749 ISSN 0066 4146 a b Sandage A R Kron R G amp Longair M S 1995 The Deep Universe Saas Fee Advanced Course 23 Lecture Notes 1993 Swiss Society for Astrophysics and Astronomy Vol 23 Springer Science amp Business Media a b Chamba Nushkia 2020 A historical perspective on the concept of galaxy size Research Notes of the American Astronomical Society 4 7 117 arXiv 2010 07946 Bibcode 2020RNAAS 4 117C doi 10 3847 2515 5172 aba951 S2CID 223953458 Mushotzky Richard Measuring Galaxy Luminosities PDF ASTR620 Galaxies University of Maryland Astronomy Department Retrieved February 14 2024 Sparke amp Gallagher 2000 1 3 1 Fouque P Paturel G September 1985 Standard photometric diameters of galaxies II Reduction of the ESO UGC MCG catalogue Astronomy and Astrophysics 150 2 192 204 Bibcode 1985A amp A 150 192F By Name NASA IPAC Extragalactic Database ned ipac caltech edu Retrieved March 8 2024 Goodwin S P John Gribbin and M A Hendry The relative size of the Milky Way The Observatory 118 1998 201 208 By Name NASA IPAC Extragalactic Database ned ipac caltech edu Retrieved March 8 2024 By Name NASA IPAC Extragalactic Database ned ipac caltech edu Retrieved March 8 2024 De Vaucouleurs Gerard 1948 Recherches sur les Nebuleuses Extragalactiques Annales d Astrophysique 11 247 Bibcode 1948AnAp 11 247D Fish Robert A 1963 Significance of a Luminosity Concentration Law in Elliptical Galaxies The Astronomical Journal 68 72 Bibcode 1963AJ 68R 72F doi 10 1086 109075 Sersic J L 1968 On the formation of galaxies by fragmentation Bulletin of the Astronomical Institutes of Czechoslovakia 19 105 Bibcode 1968BAICz 19 105S Conselice Christopher J Bershady Matthew A Jangren Anna 2000 The Asymmetry of Galaxies Physical Morphology for Nearby and High Redshift Galaxies The Astrophysical Journal 529 2 886 910 arXiv astro ph 9907399 Bibcode 2000ApJ 529 886C doi 10 1086 308300 S2CID 118962524 Burgarella D Buat V Donas J Milliard B Chapelon S 2001 The ultraviolet visibility and quantitative morphology of galactic disks at low and high redshift Astronomy amp Astrophysics 369 2 421 431 arXiv astro ph 0101344 Bibcode 2001A amp A 369 421B doi 10 1051 0004 6361 20010107 S2CID 858029 Lauberts Andris Valentijn Edwin A 1989 The surface photometry catalogue of the ESO Uppsala galaxies Bibcode 1989spce book L Petrosian V 1976 Surface Brightness and Evolution of Galaxies The Astrophysical Journal 210 L53 Bibcode 1976ApJ 209L 1P doi 10 1086 182301 SkyServer Algorithms a b Graham Alister W Driver Simon P Petrosian Vahe Conselice Christopher J Bershady Matthew A Crawford Steven M Goto Tomotsugu 2005 Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii The Astronomical Journal 130 4 1535 1544 arXiv astro ph 0504287 Bibcode 2005AJ 130 1535G doi 10 1086 444475 S2CID 11517686 Circular and Fixed Elliptical Apertures The Petrosian and Isophotal Photometry About NED NASA IPAC Extragalactic Database ned ipac caltech edu a b Jarrett T H Chester T Cutri R Schneider S E Huchra J P 2003 The 2MASS Large Galaxy Atlas The Astronomical Journal 125 2 525 554 Bibcode 2003AJ 125 525J doi 10 1086 345794 S2CID 117784410 a b Beck Rainer 2007 Galactic magnetic fields Scholarpedia 2 8 2411 Bibcode 2007SchpJ 2 2411B doi 10 4249 scholarpedia 2411 Construction Secrets of a Galactic Metropolis www eso org ESO Press Release Archived from the original on March 24 2021 Retrieved October 15 2014 a b Protogalaxies Harvard Smithsonian Center for Astrophysics November 18 1999 Archived from the original on March 25 2008 Retrieved January 10 2007 Firmani C Avila Reese V 2003 Physical processes behind the morphological Hubble sequence Revista Mexicana de Astronomia y Astrofisica 17 107 120 arXiv astro ph 0303543 Bibcode 2003RMxAC 17 107F Skibba Ramin January 10 2023 Astronomers May Have Just Spotted the Universe s First Galaxies NASA s new JWST space telescope has revealed some cosmic surprises including galaxies that might have assembled earlier than previously thought Wired Retrieved January 10 2023 McMahon R 2006 Astronomy Dawn after the dark age Nature 443 7108 151 2 Bibcode 2006Natur 443 151M doi 10 1038 443151a PMID 16971933 S2CID 28977650 Wall Mike December 12 2012 Ancient Galaxy May Be Most Distant Ever Seen Space com Archived from the original on May 4 2019 Retrieved December 12 2012 Cosmic Detectives The European Space Agency ESA April 2 2013 Archived from the original on February 11 2019 Retrieved April 15 2013 HubbleSite NewsCenter Astronomers Set a New Galaxy Distance Record 05 05 2015 Introduction hubblesite org Archived from the original on December 9 2016 Retrieved May 7 2015 This Galaxy Far Far Away Is the Farthest One Yet Found Space com May 5 2015 Archived from the original on October 2 2015 Retrieved May 7 2015 Astronomers unveil the farthest galaxy Archived from the original on September 11 2017 Retrieved May 7 2015 Overbye Dennis May 5 2015 Astronomers Measure Distance to Farthest Galaxy Yet The New York Times ISSN 0362 4331 Archived from the original on April 13 2019 Retrieved May 7 2015 Oesch P A van Dokkum P G Illingworth G D Bouwens R J Momcheva I Holden B Roberts Borsani G W Smit R Franx M February 18 2015 A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z 7 730 using Keck MOSFIRE The Astrophysical Journal 804 2 L30 arXiv 1502 05399 Bibcode 2015ApJ 804L 30O doi 10 1088 2041 8205 804 2 L30 S2CID 55115344 Atkinson Nancy August 17 2022 Here s the Largest Image JWST Has Taken So Far Universe Today Retrieved August 18 2022 Whitt Kelly Kizer August 18 2022 Webb s largest image of galaxies yet Earth amp Sky Retrieved August 19 2022 Overbye Dennis August 23 2022 How the Webb Telescope Expanded My Universe As new images of Jupiter and a galactic survey spring forth from NASA s new observatory our cosmic affairs correspondent confesses he didn t anticipate their power The New York Times Retrieved August 24 2022 a b c d Roberts Borsani G Treu T Chen W May 17 2023 The nature of an ultra faint galaxy in the cosmic dark ages seen with JWST Nature 618 7965 480 483 arXiv 2210 15639 Bibcode 2023Natur 618 480R doi 10 1038 s41586 023 05994 w PMID 37198479 S2CID 258741077 a b c Ober Holly June 1 2023 Astrophysicists confirm the faintest galaxy ever seen in the early universe Phys org Retrieved June 12 2023 Signatures of the Earliest Galaxies Archived from the original on August 6 2020 Retrieved September 15 2015 Eggen O J Lynden Bell D Sandage A R 1962 Evidence from the motions of old stars that the Galaxy collapsed The Astrophysical Journal 136 748 Bibcode 1962ApJ 136 748E doi 10 1086 147433 Searle L Zinn R 1978 Compositions of halo clusters and the formation of the galactic halo The Astrophysical Journal 225 1 357 379 Bibcode 1978ApJ 225 357S doi 10 1086 156499 Heger A Woosley S E 2002 The Nucleosynthetic Signature of Population III The Astrophysical Journal 567 1 532 543 arXiv astro ph 0107037 Bibcode 2002ApJ 567 532H doi 10 1086 338487 S2CID 16050642 Barkana R Loeb A 2001 In the beginning the first sources of light and the reionization of the Universe PDF Physics Reports Submitted manuscript 349 2 125 238 arXiv astro ph 0010468 Bibcode 2001PhR 349 125B doi 10 1016 S0370 1573 01 00019 9 S2CID 119094218 Archived PDF from the original on March 14 2021 Retrieved July 25 2018 Sobral David Matthee Jorryt Darvish Behnam Schaerer Daniel Mobasher Bahram Rottgering Huub J A Santos Sergio Hemmati Shoubaneh June 4 2015 Evidence for POPIII like Stellar Populations in the Most Luminous LYMAN a Emitters at the Epoch of Re ionisation Spectroscopic Confirmation The Astrophysical Journal 808 2 139 arXiv 1504 01734 Bibcode 2015ApJ 808 139S doi 10 1088 0004 637x 808 2 139 S2CID 18471887 Overbye Dennis June 17 2015 Traces of Earliest Stars That Enriched Cosmos Are Spied The New York Times Archived from the original on June 29 2019 Retrieved June 17 2015 Overbye Dennis January 5 2024 The Early Universe Was Bananas Images from the Webb telescope suggest that newborn galaxies look weirder than expected Exactly how screwy was physics at the dawn of time The New York Times Archived from the original on January 5 2024 Retrieved January 6 2024 Pandya Viraj et al October 23 2023 Galaxies Going Bananas Inferring the 3D Geometry of High Redshift Galaxies with JWST CEERS arXiv 2310 15232 astro ph GA O Callaghan Jonathan December 6 2022 Astronomers Grapple with JWST s Discovery of Early Galaxies Researchers are convinced the James Webb Space Telescope has glimpsed an unexpected population of galaxies in the early universe Now they re trying to decide what this means for our understanding of the cosmos Scientific American Retrieved December 6 2022 Simulations Show How Growing Black Holes Regulate Galaxy Formation Carnegie Mellon University February 9 2005 Archived from the original on June 4 2012 Retrieved January 7 2007 Massey R April 21 2007 Caught in the act forming galaxies captured in the young Universe Royal Astronomical Society Archived from the original on November 15 2013 Retrieved April 20 2007 Noguchi M 1999 Early Evolution of Disk Galaxies Formation of Bulges in Clumpy Young Galactic Disks The Astrophysical Journal 514 1 77 95 arXiv astro ph 9806355 Bibcode 1999ApJ 514 77N doi 10 1086 306932 S2CID 17963236 Baugh C Frenk C May 1999 How are galaxies made PhysicsWeb Archived from the original on April 26 2007 Retrieved January 16 2007 Gonzalez G 1998 The Stellar Metallicity Planet Connection Brown dwarfs and extrasolar planets Proceedings of a workshop p 431 Bibcode 1998ASPC 134 431G Moskowitz Clara September 25 2012 Hubble Telescope Reveals Farthest View Into Universe Ever Space com Archived from the original on May 5 2020 Retrieved September 26 2012 Rojas Randall R Vogeley Michael S Hoyle Fiona Brinkmann Jon May 10 2005 Spectroscopic Properties of Void Galaxies in the Sloan Digital Sky Survey The Astrophysical Journal 624 2 571 585 arXiv astro ph 0409074 doi 10 1086 428476 ISSN 0004 637X Conselice C J February 2007 The Universe s Invisible Hand Scientific American Vol 296 no 2 pp 35 41 Bibcode 2007SciAm 296b 34C doi 10 1038 scientificamerican0207 34 Ford H et al April 30 2002 The Mice NGC 4676 Colliding Galaxies With Tails of Stars and Gas Hubble News Desk Archived from the original on September 7 2016 Retrieved May 8 2007 Struck C 1999 Galaxy Collisions Physics Reports 321 1 3 1 137 arXiv astro ph 9908269 Bibcode 1999PhR 321 1S doi 10 1016 S0370 1573 99 00030 7 S2CID 119369136 Buser R 2000 The Formation and Early Evolution of the Milky Way Galaxy Science 287 5450 69 74 Bibcode 2000Sci 287 69B doi 10 1126 science 287 5450 69 PMID 10615051 Kruijssen J M Diederik Pfeffer Joel L Chevance Melanie Bonaca Ana Trujillo Gomez Sebastian Bastian Nate Reina Campos Marta Crain Robert A Hughes Meghan E October 2020 Kraken reveals itself the merger history of the Milky Way reconstructed with the E MOSAICS simulations Monthly Notices of the Royal Astronomical Society 498 2 2472 2491 arXiv 2003 01119 doi 10 1093 mnras staa2452 Archived from the original on November 16 2020 Retrieved November 15 2020 Young Monica November 13 2020 Star Clusters reveal the Kraken in the Milky Way s Past Sky and Telescope Archived from the original on November 15 2020 Retrieved November 15 2020 Panter B Jimenez R Heavens A F Charlot S 2007 The star formation histories of galaxies in the Sloan Digital Sky Survey Monthly Notices of the Royal Astronomical Society 378 4 1550 1564 arXiv astro ph 0608531 Bibcode 2007MNRAS 378 1550P doi 10 1111 j 1365 2966 2007 11909 x S2CID 15174718 Kennicutt R C Jr Tamblyn P Congdon C E 1994 Past and future star formation in disk galaxies The Astrophysical Journal 435 1 22 36 Bibcode 1994ApJ 435 22K doi 10 1086 174790 Knapp G R 1999 Star Formation in Early Type Galaxies Vol 163 Astronomical Society of the Pacific p 119 arXiv astro ph 9808266 Bibcode 1999ASPC 163 119K ISBN 978 1 886733 84 8 OCLC 41302839 Archived from the original on March 24 2021 Retrieved July 25 2018 a b Adams Fred Laughlin Greg July 13 2006 The Great Cosmic Battle Astronomical Society of the Pacific Archived from the original on July 31 2012 Retrieved January 16 2007 Cosmic Murder Mystery Solved Galaxies Are Strangled to Death Space com May 13 2015 Archived from the original on March 24 2021 Retrieved May 14 2015 Pobojewski S January 21 1997 Physics offers glimpse into the dark side of the Universe University of Michigan Archived from the original on June 4 2012 Retrieved January 13 2007 Groups amp Clusters of Galaxies NASA Chandra Archived from the original on July 7 2012 Retrieved January 15 2007 Ricker P When Galaxy Clusters Collide San Diego Supercomputer Center Archived from the original on August 5 2012 Retrieved August 27 2008 Dahlem M November 24 2006 Optical and radio survey of Southern Compact Groups of galaxies University of Birmingham Astrophysics and Space Research Group Archived from the original on June 13 2007 Retrieved January 15 2007 Ponman T February 25 2005 Galaxy Systems Groups University of Birmingham Astrophysics and Space Research Group Archived from the original on February 15 2009 Retrieved January 15 2007 Girardi M Giuricin G 2000 The Observational Mass Function of Loose Galaxy Groups The Astrophysical Journal 540 1 45 56 arXiv astro ph 0004149 Bibcode 2000ApJ 540 45G doi 10 1086 309314 S2CID 14059401 Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen ESA Hubble Press Release Archived from the original on June 12 2018 Retrieved January 22 2015 Dubinski J 1998 The Origin of the Brightest Cluster Galaxies The Astrophysical Journal 502 2 141 149 arXiv astro ph 9709102 Bibcode 1998ApJ 502 141D doi 10 1086 305901 S2CID 3137328 Archived from the original on May 14 2011 Retrieved January 16 2007 ATLASGAL Survey of Milky Way Completed Archived from the original on March 24 2021 Retrieved March 7 2016 Bahcall N A 1988 Large scale structure in the Universe indicated by galaxy clusters Annual Review of Astronomy and Astrophysics 26 1 631 686 Bibcode 1988ARA amp A 26 631B doi 10 1146 annurev aa 26 090188 003215 Mandolesi N et al 1986 Large scale homogeneity of the Universe measured by the microwave background Letters to Nature 319 6056 751 753 Bibcode 1986Natur 319 751M doi 10 1038 319751a0 S2CID 4349689 Horvath Istvan Bagoly Zsolt Hakkila Jon Toth L Viktor 2015 New data support the existence of the Hercules Corona Borealis Great Wall Astronomy amp Astrophysics 584 A48 arXiv 1510 01933 Bibcode 2015A amp A 584A 48H doi 10 1051 0004 6361 201424829 S2CID 56073380 Horvath Istvan Bagoly Zsolt Hakkila Jon Toth L Viktor 2014 Anomalies in the GRB spatial distribution Proceedings of Science 78 arXiv 1507 05528 Bibcode 2014styd confE 78H doi 10 22323 1 233 0078 Balazs L G Bagoly Z Hakkila J E Horvath I Kobori J Racz I Toth L V 2015 A giant ring like structure at 0 78 lt z lt 0 86 displayed by GRBs Monthly Notices of the Royal Astronomical Society 452 3 2236 arXiv 1507 00675 Bibcode 2015MNRAS 452 2236B doi 10 1093 mnras stv1421 S2CID 109936564 van den Bergh S 2000 Updated Information on the Local Group Publications of the Astronomical Society of the Pacific 112 770 529 536 arXiv astro ph 0001040 Bibcode 2000PASP 112 529V doi 10 1086 316548 S2CID 1805423 Tully R B 1982 The Local Supercluster The Astrophysical Journal 257 389 422 Bibcode 1982ApJ 257 389T doi 10 1086 159999 Tempel Elmo September 1 2014 Cosmology Meet the Laniakea supercluster Nature 513 7516 41 42 Bibcode 2014Natur 513 41T doi 10 1038 513041a PMID 25186896 Rieke George Henry 2012 Measuring the universe a multiwavelength perspective Cambridge Cambridge university press ISBN 978 0 521 76229 8 Near Mid amp Far Infrared IPAC NASA Archived from the original on December 30 2006 Retrieved January 2 2007 Fraknoi et al 2023 pp 144 202 The Effects of Earth s Upper Atmosphere on Radio Signals NASA Archived from the original on May 29 2012 Retrieved August 10 2006 Max Planck Society December 14 2006 Giant Radio Telescope Imaging Could Make Dark Matter Visible ScienceDaily Archived from the original on July 3 2017 Retrieved January 2 2007 NASA Telescope Sees Black Hole Munch on a Star NASA December 5 2006 Archived from the original on June 4 2012 Retrieved January 2 2007 Dunn R August 9 2005 An Introduction to X ray Astronomy Institute of Astronomy X Ray Group Archived from the original on July 17 2012 Retrieved January 2 2007 Webb reveals structure in 19 spiral galaxies ESA January 29 2024 Retrieved January 30 2024 BibliographyBelkora L 2003 Minding the Heavens the Story of our Discovery of the Milky Way CRC Press ISBN 978 0 7503 0730 7 Archived from the original on March 24 2021 Retrieved July 25 2018 Bertin G Lin C C 1996 Spiral Structure in Galaxies a Density Wave Theory MIT Press ISBN 978 0 262 02396 2 Archived from the original on March 24 2021 Retrieved July 25 2018 Binney J Merrifield M 1998 Galactic Astronomy Princeton University Press ISBN 978 0 691 00402 0 OCLC 39108765 Archived from the original on March 24 2021 Retrieved July 25 2018 Dickinson T 2004 The Universe and Beyond 4th ed Firefly Books ISBN 978 1 55297 901 3 OCLC 55596414 Fraknoi A et al 2023 Astronomy 2e OpenStax ISBN 978 1 951693 50 3 Heidarzadeh T 2008 A History of Physical Theories of Comets from Aristotle to Whipple Springer ISBN 978 1 4020 8322 8 Archived from the original on March 24 2021 Retrieved July 25 2018 Mo H van den Bosch F White S 2010 Galaxy Formation and Evolution Cambridge University Press ISBN 978 0 521 85793 2 Archived from the original on March 24 2021 Retrieved July 25 2018 Kepple G R Sanner G W 1998 The Night Sky Observer s Guide Volume 1 Willmann Bell ISBN 978 0 943396 58 3 Archived from the original on March 24 2021 Retrieved July 25 2018 Merritt D 2013 Dynamics and Evolution of Galactic Nuclei Princeton University Press ISBN 978 1 4008 4612 2 Archived from the original on March 24 2021 Retrieved July 25 2018 Mohamed M 2000 Great Muslim Mathematicians Penerbit UTM ISBN 978 983 52 0157 8 OCLC 48759017 Archived from the original on March 24 2021 Retrieved July 25 2018 Paul E R 1993 The Milky Way Galaxy and Statistical Cosmology 1890 1924 Cambridge University Press ISBN 978 0 521 35363 2 Archived from the original on March 24 2021 Retrieved July 25 2018 Sparke L S Gallagher J S III 2000 Galaxies in the Universe An Introduction Cambridge University Press ISBN 978 0 521 59740 1 Archived from the original on March 24 2021 Retrieved July 25 2018 Van den Bergh S 1998 Galaxy Morphology and Classification Cambridge University Press ISBN 978 0 521 62335 3 Archived from the original on March 24 2021 Retrieved July 25 2018 Waller W H Hodge P W 2003 Galaxies and the Cosmic Frontier Harvard University Press ISBN 978 0 674 01079 6 External linksGalaxy at Wikipedia s sister projects nbsp Definitions from Wiktionary nbsp Media from Commons nbsp News from Wikinews nbsp Quotations from Wikiquote nbsp Textbooks from Wikibooks nbsp Resources from Wikiversity NASA IPAC Extragalactic Database NED NED Distances Galaxies on In Our Time at the BBC An Atlas of The Universe Galaxies Information and amateur observations Citizen science galaxy classification project see Galaxy zoo How many galaxies are in our universe Archived August 21 2015 at the Wayback Machine A Flight Through the Universe by the Sloan Digital Sky Survey animated video from Berkely Lab Portals nbsp Astronomy nbsp Stars nbsp Spaceflight nbsp Outer space nbsp Solar System Retrieved from https en wikipedia org w index php title Galaxy amp oldid 1217675380, wikipedia, wiki, book, books, library,

article

, read, download, free, free download, mp3, video, mp4, 3gp, jpg, jpeg, gif, png, picture, music, song, movie, book, game, games.