A galaxy group[2] or group of galaxies[3] (GrG[4]) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 1010 times the luminosity of the Sun); collections of galaxies larger than groups that are first-order clustering are called galaxy clusters.[5] The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies.
Four of the seven members of galaxy group HCG 16[1]
Groups of galaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc).[NB 1] Their mass is approximately 1013solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups.[7]
Groups are the most common structures of galaxies in the universe, accounting for at least 50% of the galaxies in the local universe. Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies.[8] In the local universe, about half of the groups exhibit diffuse X-ray emissions from their intracluster media. Those that emit X-rays appear to have early-type galaxies as members. The diffuse X-ray emissions come from zones within the inner 10–50% of the groups' virial radius, generally 50–500 kpc.[9]
Typesedit
There are several subtypes of groups.
Compact groupsedit
A compact group consists of a small number of galaxies, typically around five, in close proximity and relatively isolated from other galaxies and formations.[10] The first compact group to be discovered was Stephan's Quintet, found in 1877.[11] Stephan's Quintet is named for a compact group of four galaxies plus an unassociated foreground galaxy.[10] Astronomer Paul Hickson created a catalogue of such groups in 1982, the Hickson Compact Groups.[12]
Compact groups of galaxies readily show the effect of dark matter, as the visible mass is greatly less than that needed to gravitationally hold the galaxies together in a bound group. Compact galaxy groups are also not dynamically stable over Hubble time, thus showing that galaxies evolve by merger, over the timescale of the age of the universe.[10]
Fossil groupsedit
Fossil galaxy groups, fossil groups, or fossil clusters are believed to be the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. Galaxies within a group interact and merge. The physical process behind this galaxy-galaxy merger is dynamical friction. The time-scales for dynamical friction on luminous (or L*) galaxies suggest that fossil groups are old, undisturbed systems that have seen little infall of L* galaxies since their initial collapse. Fossil groups are thus an important laboratory for studying the formation and evolution of galaxies and the intragroup medium in an isolated system. Fossil groups may still contain unmerged dwarf galaxies, but the more massive members of the group have condensed into the central galaxy.[9][10] This hypothesis is supported by studies of computer simulations of cosmological volumes.[13]
Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters.[15] These contain galaxies and protogalaxies embedded in dark matter haloes that are in the process of fusing into group-formations of singular dark matter halos.[16]
^Mike Irwin. "The Local Group". Retrieved 2009-11-07.
^UTK Physics Dept. "Groups of Galaxies". University of Tennessee, Knoxville. Retrieved September 27, 2012.
^Muñoz, R. P.; Motta, V.; Verdugo, T.; Garrido, F.; et al. (11 December 2012). "Dynamical analysis of strong-lensing galaxy groups at intermediate redshift". Astronomy & Astrophysics (published April 2013). 552: 18. arXiv:1212.2624. Bibcode:2013A&A...552A..80M. doi:10.1051/0004-6361/201118513. S2CID 17865754. A80.
^ abMulchaey, John S. (22 September 2000). "X-ray Properties of Groups of Galaxies". Annual Review of Astronomy and Astrophysics (published 2000). 38: 289–335. arXiv:astro-ph/0009379. Bibcode:2000ARA&A..38..289M. doi:10.1146/annurev.astro.38.1.289. S2CID 1427929.
^ abcdPaul Hickson (1997). "Compact Groups of Galaxies". Annual Review of Astronomy and Astrophysics. 35: 357–388. arXiv:astro-ph/9710289. Bibcode:1997ARA&A..35..357H. doi:10.1146/annurev.astro.35.1.357. S2CID 18870169.
^M. Stephan (April 1877). "Nebulæ (new) discovered and observed at the observatory of Marseilles, 1876 and 1877, M. Stephan". Monthly Notices of the Royal Astronomical Society. 37 (6): 334. Bibcode:1877MNRAS..37..334S. doi:10.1093/mnras/37.6.334.
^Hickson, Paul (April 1982). "Systematic properties of compact groups of galaxies". Astrophysical Journal, Part 1. 255: 382–391. Bibcode:1982ApJ...255..382H. doi:10.1086/159838.
^Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations
^An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482
^Yujin Yang (2008). Testing Both Modes of Galaxy Formation: A Closer Look at Galaxy Mergers and Gas Accretion. p. 205. ISBN9780549692300. {{cite book}}: |work= ignored (help)
^C. Diener; S. J. Lilly; C. Knobel; G. Zamorani; et al. (9 October 2012). "Proto-groups at 1.8<z<3 in the zCOSMOS-deep sample". The Astrophysical Journal (published March 2013). 765 (2): 11. arXiv:1210.2723. Bibcode:2013ApJ...765..109D. doi:10.1088/0004-637X/765/2/109. S2CID 17700696. 109.{{cite journal}}: CS1 maint: multiple names: authors list (link)
galaxy, group, galaxy, group, group, galaxies, aggregation, galaxies, comprising, about, fewer, gravitationally, bound, members, each, least, luminous, milky, about, 1010, times, luminosity, collections, galaxies, larger, than, groups, that, first, order, clus. A galaxy group 2 or group of galaxies 3 GrG 4 is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members each at least as luminous as the Milky Way about 1010 times the luminosity of the Sun collections of galaxies larger than groups that are first order clustering are called galaxy clusters 5 The groups and clusters of galaxies can themselves be clustered into superclusters of galaxies Four of the seven members of galaxy group HCG 16 1 The Milky Way galaxy is part of a group of galaxies called the Local Group 6 Contents 1 Characteristics 2 Types 2 1 Compact groups 2 2 Fossil groups 2 3 Proto groups 3 List 4 See also 5 Notes 6 ReferencesCharacteristics editGroups of galaxies are the smallest aggregates of galaxies They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs Mpc NB 1 Their mass is approximately 1013 solar masses The spread of velocities for the individual galaxies is about 150 km s However this definition should be used as a guide only as larger and more massive galaxy systems are sometimes classified as galaxy groups 7 Groups are the most common structures of galaxies in the universe accounting for at least 50 of the galaxies in the local universe Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies 8 In the local universe about half of the groups exhibit diffuse X ray emissions from their intracluster media Those that emit X rays appear to have early type galaxies as members The diffuse X ray emissions come from zones within the inner 10 50 of the groups virial radius generally 50 500 kpc 9 Types editThere are several subtypes of groups Compact groups edit A compact group consists of a small number of galaxies typically around five in close proximity and relatively isolated from other galaxies and formations 10 The first compact group to be discovered was Stephan s Quintet found in 1877 11 Stephan s Quintet is named for a compact group of four galaxies plus an unassociated foreground galaxy 10 Astronomer Paul Hickson created a catalogue of such groups in 1982 the Hickson Compact Groups 12 Compact groups of galaxies readily show the effect of dark matter as the visible mass is greatly less than that needed to gravitationally hold the galaxies together in a bound group Compact galaxy groups are also not dynamically stable over Hubble time thus showing that galaxies evolve by merger over the timescale of the age of the universe 10 Fossil groups edit Fossil galaxy groups fossil groups or fossil clusters are believed to be the end result of galaxy merging within a normal galaxy group leaving behind the X ray halo of the progenitor group Galaxies within a group interact and merge The physical process behind this galaxy galaxy merger is dynamical friction The time scales for dynamical friction on luminous or L galaxies suggest that fossil groups are old undisturbed systems that have seen little infall of L galaxies since their initial collapse Fossil groups are thus an important laboratory for studying the formation and evolution of galaxies and the intragroup medium in an isolated system Fossil groups may still contain unmerged dwarf galaxies but the more massive members of the group have condensed into the central galaxy 9 10 This hypothesis is supported by studies of computer simulations of cosmological volumes 13 The closest fossil group to the Milky Way is NGC 6482 an elliptical galaxy at a distance of approximately 180 million light years located in the constellation of Hercules 14 Proto groups edit Proto groups are groups that are in the process of formation They are the smaller form of protoclusters 15 These contain galaxies and protogalaxies embedded in dark matter haloes that are in the process of fusing into group formations of singular dark matter halos 16 List editMain article List of galaxy groups and clusters Notable groups Group NotesLocal Group The group where the Milky Way including the Earth is locatedStephan s Quintet One of the most photogenic groupsRobert s Quartet Another very notable groupBullet Group The merging group exhibits separation of dark matter from normal matterThis lists some of the most notable groups for more groups see the list article See also editIllustris projectNotes edit see 1022 m for distance comparisonsReferences edit Hubble views a bizarre cosmic quartet Retrieved 19 June 2015 Barbel Koribalski 2004 The NGC 6221 15 Galaxy Group Hartmut Frommert amp Christine Kronberg Groups and Clusters of Galaxies with Messier objects SEDS Object classification in SIMBAD SIMBAD November 2013 Sparke L S amp Gallagher J S 2007 Galaxies in the Universe an Introduction 2nd ed Cambridge University Press p 278 ISBN 9780521671866 Mike Irwin The Local Group Retrieved 2009 11 07 UTK Physics Dept Groups of Galaxies University of Tennessee Knoxville Retrieved September 27 2012 Munoz R P Motta V Verdugo T Garrido F et al 11 December 2012 Dynamical analysis of strong lensing galaxy groups at intermediate redshift Astronomy amp Astrophysics published April 2013 552 18 arXiv 1212 2624 Bibcode 2013A amp A 552A 80M doi 10 1051 0004 6361 201118513 S2CID 17865754 A80 a b Mulchaey John S 22 September 2000 X ray Properties of Groups of Galaxies Annual Review of Astronomy and Astrophysics published 2000 38 289 335 arXiv astro ph 0009379 Bibcode 2000ARA amp A 38 289M doi 10 1146 annurev astro 38 1 289 S2CID 1427929 a b c d Paul Hickson 1997 Compact Groups of Galaxies Annual Review of Astronomy and Astrophysics 35 357 388 arXiv astro ph 9710289 Bibcode 1997ARA amp A 35 357H doi 10 1146 annurev astro 35 1 357 S2CID 18870169 M Stephan April 1877 Nebulae new discovered and observed at the observatory of Marseilles 1876 and 1877 M Stephan Monthly Notices of the Royal Astronomical Society 37 6 334 Bibcode 1877MNRAS 37 334S doi 10 1093 mnras 37 6 334 Hickson Paul April 1982 Systematic properties of compact groups of galaxies Astrophysical Journal Part 1 255 382 391 Bibcode 1982ApJ 255 382H doi 10 1086 159838 Dependency of halo concentration on mass redshift and fossilness in Magneticum hydrodynamic simulations An old galaxy group Chandra X ray observations of the nearby fossil group NGC 6482 Yujin Yang 2008 Testing Both Modes of Galaxy Formation A Closer Look at Galaxy Mergers and Gas Accretion p 205 ISBN 9780549692300 a href Template Cite book html title Template Cite book cite book a work ignored help C Diener S J Lilly C Knobel G Zamorani et al 9 October 2012 Proto groups at 1 8 lt z lt 3 in the zCOSMOS deep sample The Astrophysical Journal published March 2013 765 2 11 arXiv 1210 2723 Bibcode 2013ApJ 765 109D doi 10 1088 0004 637X 765 2 109 S2CID 17700696 109 a href Template Cite journal html title Template Cite journal cite journal a CS1 maint multiple names authors list link Portals nbsp Stars nbsp Spaceflight nbsp Outer space nbsp Solar System Retrieved from https en wikipedia org w index php title Galaxy group amp oldid 1141312558 Fossil groups, wikipedia, wiki, book, books, library,
