Galaxy group

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Four of the seven members of galaxy group HCG 16.[1]

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.

Our Milky Way Galaxy is part of a group of galaxies called the Local Group.[6]


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 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, comprising 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]


There are several subtypes of groups.

Compact Groups[edit]

Compact groups are small groups in closely grouped in a small area. They typically have around 5 galaxies in close proximity relatively isolated from other galaxies and formations.[10] The first discovered, Stephan's Quintet, was discovered in 1877.[11] Though Stephan's Quintet itself is a group of 4 and 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 dynamically bind the galaxies into 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]

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.[13]


Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters.[14] 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.[15]


Notable groups
Group Notes
Local Group The nearest group to Earth, and the one to which we belong
Stephan's Quintet One of the most photogenic groups
Bullet Group This merging group exhibits separation of dark matter from normal matter
This lists some of the most notable groups; for more groups, see the list article.

See also[edit]


  1. ^ see 1022 m for distance comparisons


  1. ^ "Hubble views a bizarre cosmic quartet". Retrieved 19 June 2015. 
  2. ^ Bärbel Koribalski (2004). "The NGC 6221/15 Galaxy Group". 
  3. ^ Hartmut Frommert & Christine Kronberg. "Groups and Clusters of Galaxies with Messier objects". SEDS. 
  4. ^ "Object classification in SIMBAD". SIMBAD. November 2013. 
  5. ^ L.S. Sparke & J.S. Gallagher (2007). Galaxies in the Universe: an Introduction (2nd ed.). Cambridge University Press. p. 278. ISBN 9780521671866. 
  6. ^ Mike Irwin. "The Local Group". Retrieved 2009-11-07. 
  7. ^ UTK Physics Dept. "Groups of Galaxies". University of Tennessee, Knoville. Retrieved September 27, 2012. 
  8. ^ 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 (April 2013) 552: 18. arXiv:1212.2624. Bibcode:2013A&A...552A..80M. doi:10.1051/0004-6361/201118513. A80. 
  9. ^ a b Mulchaey, John S. (22 September 2000). "X-ray Properties of Groups of Galaxies". Annual Review of Astronomy and Astrophysics (2000) 38: 289–335. arXiv:astro-ph/0009379. Bibcode:2000ARA&A..38..289M. doi:10.1146/annurev.astro.38.1.289. 
  10. ^ 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&A..35..357H. doi:10.1146/annurev.astro.35.1.357. 
  11. ^ 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: 334. Bibcode:1877MNRAS..37..334S. doi:10.1093/mnras/37.6.334. 
  12. ^ 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. 
  13. ^ An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482
  14. ^ Yujin Yang (2008). Testing Both Modes of Galaxy Formation: A Closer Look at Galaxy Mergers and Gas Accretion. University of Arizona (ProQuest). p. 205. ISBN 9780549692300. 
  15. ^ 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 (March 2013) 765 (2): 11. arXiv:1210.2723. Bibcode:2013ApJ...765..109D. doi:10.1088/0004-637X/765/2/109. 109.