A galaxy group or group of galaxies is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 10 times the luminosity of the Sun); collections of galaxies larger than groups that are first-order clustering are called galaxy clusters. The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies.
Types[]
Stephans Quitet, a classic example of a compact group. (Taken by HST)
Compact 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. Stephan's Quintet is named for a compact group of four galaxies plus an unassociated foreground galaxy. Astronomer Paul Hickson created a catalogue of such groups in 1982, the Hickson Compact Groups.
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 Groups[]
Fossil groups tend to consist of a large, isolated elliptical galaxy embedded in an extended halo of X-ray emitting gas the size of a galaxy group. They are thought to result from the extensive merger of all the galaxies contained within a small group, with the extended X-ray halo providing strong evidence for the group origin.
The central elliptical galaxy in a fossil group is as bright as a brightest cluster galaxy, but does not possess the extended stellar halo often associated with such cluster galaxies.
The large elliptical galaxy NGC 4555 is a good example as in X-ray light displays a large halo which heavily supports the concept of Fossil Groups.
Protogroups[]
Protogroups are groups of galaxies that are in the process of formation. They are a smaller form of protoclusters.
Lists[]
A list of Galaxy Groups can be found under the category, Galaxy Clusters at the bottom of the page under the Categories tab.