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A galaxy group or group of galaxies ( GrG) is an aggregation of Galaxy comprising about 50 or fewer gravitationally bound members, each at least as Luminosity 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 . (2025). 9780521671866, Cambridge University Press. ISBN 9780521671866 The groups and clusters of galaxies can themselves be clustered, into of galaxies.
The Milky Way galaxy is part of a group of galaxies called the Local Group.
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. 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.
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.
The closest fossil group to the Milky Way is NGC 6482, an elliptical galaxy at a distance of approximately 180 million located in the constellation of Hercules. An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482
Fossil groups
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. Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations
Proto-groups
Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters. These contain galaxies and protogalaxy embedded in dark matter haloes that are in the process of fusing into group-formations of singular dark matter halos.
List
+ Notable groups Local Group The group where the Milky Way, including the Earth, is located Stephan's Quintet One of the most photogenic groups Robert's Quartet Another very notable group Bullet Group The 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.
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