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 galaxy M96 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.
The Local Group and its Neighbours
The Milky Way Galaxy belongs to a rather less eventful galaxy group, known as the Local Group. The Local Group is relatively typical of such structures; it contains three galaxies of significant mass (the Andromeda, Milky Way and Triangulum Galaxies) accompanied by a swarm of more than forty smaller galaxies. The Local Group occupies a region spanning a diameter of about five million light years, with the Milky Way lying near the centre of this region.
Beyond the Local Group, the nearest galaxy group to the Milky Way is the IC 342/Maffei Group, which straddles the border between Camelopardalis and Cassiopeia. Like the Local Group, the IC 342/Maffei Group is centred around two massive galaxies (IC 342 and Maffei 1). Unlike the Local Group, its less massive members are divided into two separate subgroups, each clustering around one of the group's two major galaxies. The entire structure lies some ten million light years from the Milky Way.
Other groups within twenty million light years of the Milky Way include the M81 Group, the Centaurus A/M83 Group, the Sculptor Group, the Canes Venatici I Group and the NGC 1023 Group. Together with the Local Group itself and the IC 342/Maffei Group, all of these galaxy groups (and many others besides) form a constituent part of the immense Virgo Supercluster.