One of several terms used for a set of galaxies bound by their mutual gravitational attraction. A 'group' of galaxies describes the smallest such collection, typically containing fifty or fewer members. Groupings with more than fifty members are commonly referred to as 'clusters' of galaxies, and even larger aggregations of clusters and groups are known as 'superclusters'.
Structures and Effects
Galaxy groups vary in dimensions, but their diameter rarely exceeds two megaparsecs (two million parsecs, or about 6.5 million light years). Within this area are not just the galaxies themselves, but in many cases a web of intracluster medium running between and among the member galaxies. Together, the galaxies and the intracluster medium make up about a tenth of the mass of a typical galaxy group, with the remainder deriving from dark matter.
Though a group typically contains several dozen galaxies, there are examples with rather fewer, and sometimes a group can comprise just a handful of galaxies in close interaction. Such miniature galaxy groups are known as compact groups. The first example of such a group, containing just five galaxies, was discovered in 1877 and is known as Stephan's Quintet after its discoverer, Édouard Stephan.
Being closely associated with one another, on a galactic scale, the members of galaxy groups can sometimes interact directly. This is especially common in compact groups, but other examples are known. Inside the M81 Group, for example, the interactions of M81 with two other galaxies in the group cause hydrogen gas to form arcs and loops connecting the three galaxies together (features that are only visible at radio wavelengths). Even more dramatically, within the Leo I Group, the galaxyM96 collided in the distant past with another member of its group, NGC 3384. This resulted in a stream of hydrogen and helium erupting out from the collision, which now forms the Leo Ring, a band of material some 650,000 light years across that surrounds the galaxy group at its core.
An example of a small and compact galaxy group, Robert's Quartet is a group of unusual galaxies in the central regions of Phoenix. Three of the four galaxies shown here are spirals, but each with their own peculiarities (such as the strangely extended spiral arm of NGC 92 to the northeast). The fourth member, NGC 87 in the northwest, is a particularly diffuse irregular galaxy. Imagery provided by Aladin sky atlas