Galaxy group

From Wikipedia, the free encyclopedia
Jump to: navigation, search

A galaxy group[1] or group of galaxies[2] (GrG[3]) 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.[4] 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.[5]


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.[6]

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.[7] 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.[8]


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.[9] The first discovered, Stephan's Quintet, was discovered in 1877.[10] Though Stephan's Quintet itself is a group of 4 and an unassociated foreground galaxy.[9] Astronomer Paul Hickson created a catalogue of such groups in 1982, the Hickson Compact Groups.[11]

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.[9]

Fossil Groups[edit]

Main article: Fossil galaxy group

Fossil galaxy groups are the remains of older groups of galaxies, where the luminous galaxies have merged to form an elliptical galaxy. Such systems still have an X-ray halo the size of the progenitor group, and can so be X-ray selected. Fossil groups may still contain unmerged dwarf galaxies, but the more massive members of the group have condensed into the central galaxy.[8][9]


Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters.[12] 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.[13]


See also[edit]


  1. ^ see 1022 m for distance comparisons


  1. ^ Bärbel Koribalski (2004). "The NGC 6221/15 Galaxy Group". 
  2. ^ Hartmut Frommert & Christine Kronberg. "Groups and Clusters of Galaxies with Messier objects". SEDS. 
  3. ^ "Object classification in SIMBAD". SIMBAD. November 2013. 
  4. ^ L.S. Sparke & J.S. Gallagher (2007). Galaxies in the Universe: an Introduction (2nd ed.). Cambridge University Press. p. 278. ISBN 9780521671866. 
  5. ^ Mike Irwin. "The Local Group". Retrieved 2009-11-07. 
  6. ^ UTK Physics Dept. "Groups of Galaxies". University of Tennessee, Knoville. Retrieved September 27, 2012. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ 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. 
  10. ^ 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. 
  11. ^ 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. 
  12. ^ 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. 
  13. ^ 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.