galaxy group

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).see 10²² m for distance comparisons 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.{{cite web |url= http://csep10.phys.utk.edu/astr162/lect/gclusters/groups.html |title= Groups of Galaxies |publisher= University of Tennessee, Knoxville |author= UTK Physics Dept |access-date= September 27, 2012 }}

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.{{cite journal |title= Dynamical analysis of strong-lensing galaxy groups at intermediate redshift |author= Muñoz, R. P. |display-authors= 4 |author2= Motta, V. |author3= Verdugo, T. |author4= Garrido, F. |author5= Limousin, M. |author6= Padilla, N. |author7= Foëx, G. |author8= Cabanac, R. |author9= Gavazzi, R. |author10= Barrientos, L. F. |author11= Richard, J. |date= 11 December 2012 |publication-date= April 2013 |journal= Astronomy & Astrophysics |volume= 552 |id= A80 |page= 18 |arxiv= 1212.2624 |doi= 10.1051/0004-6361/201118513 |bibcode= 2013A&A...552A..80M |s2cid= 17865754 }} 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.{{cite journal |title= X-ray Properties of Groups of Galaxies |author= Mulchaey, John S. |journal= Annual Review of Astronomy and Astrophysics |volume= 38 |pages= 289–335 |publication-date= 2000 |date= 22 September 2000 |url= http://ned.ipac.caltech.edu/level5/Sept03/Mulchaey/frames.html |arxiv= astro-ph/0009379 |doi= 10.1146/annurev.astro.38.1.289 |bibcode= 2000ARA&A..38..289M |s2cid= 1427929 }}

There are several subtypes of groups.

A compact group consists of a small number of galaxies, typically around five, in close proximity and relatively isolated from other galaxies and formations. The first compact group to be discovered was Stephan's Quintet, found in 1877.{{cite journal |author= M. Stephan |date= April 1877 |title= Nebulæ (new) discovered and observed at the observatory of Marseilles, 1876 and 1877, M. Stephan |bibcode= 1877MNRAS..37..334S |journal= Monthly Notices of the Royal Astronomical Society |volume= 37 |issue= 6 |page= 334 | doi = 10.1093/mnras/37.6.334 |doi-access= free }} Stephan's Quintet is named for a compact group of four galaxies plus an unassociated foreground galaxy.{{cite journal |title= Compact Groups of Galaxies |author= Paul Hickson |url= http://ned.ipac.caltech.edu/level5/Sept01/Hickson/Hickson_contents.html |journal= Annual Review of Astronomy and Astrophysics |volume= 35 |date= 1997 |pages= 357–388 |bibcode= 1997ARA&A..35..357H |doi= 10.1146/annurev.astro.35.1.357 |arxiv = astro-ph/9710289 |s2cid= 18870169 }} Astronomer Paul Hickson created a catalogue of such groups in 1982, the Hickson Compact Groups.{{Cite journal |author= Hickson, Paul |title= Systematic properties of compact groups of galaxies |journal= Astrophysical Journal, Part 1 |date= April 1982 |volume= 255 |pages= 382–391 |bibcode= 1982ApJ...255..382H |doi= 10.1086/159838 }}

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.

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. This hypothesis is supported by studies of computer simulations of cosmological volumes.[http://adsabs.harvard.edu/abs/2019MNRAS.486.4001R/abstract Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations]

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.[http://adsabs.harvard.edu/abs/2004MNRAS.349.1240K An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482]

Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters.{{cite book |title= Testing Both Modes of Galaxy Formation: A Closer Look at Galaxy Mergers and Gas Accretion |author= Yujin Yang |date= 2008 |page= 205 |publisher= University of Arizona |isbn= 9780549692300 }} 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.{{cite journal |title= Proto-groups at 1.8

{{main|List of galaxy groups and clusters}}

• Illustris project

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Groups