About half the galaxies in the Universe are found in groups and clusters, complexes where typically half the member galaxies are packed into a region ≲1 Mpc across. Groups and clusters no longer expand with the cosmic flow: mutual gravitational attraction is strong enough that the galaxies are moving inward, or have already passed through the core. Clusters are the denser and richer structures. Within the central megaparsec, they typically contain at least 50 luminous galaxies: those with L ≳ L * ~ 2 × 1010 L ☉. Poorer associations are called groups; they are generally less massive than ~ 1014 M ☉.
Galaxies and clusters are not simply concentrations of galaxies: the fact that a galaxy is a member significantly affects its development. The most common inhabitants of groups are spiral and irregular galaxies. The Local Group is typical; its three bright members are all spirals. Almost all the large galaxies of the Ursa Major group, featured in Figures 5.6, 5.8, and 5.23, are disk systems; there are no more than two ellipticals among the brightest 79 galaxies. Elliptical and S0 galaxies predominate in dense clusters. Curiously, both the most luminous giant ellipticals and the dwarfs with L ≲ 3 × 109 L ☉ are concentrated more strongly into clusters than the midsized ellipticals.
By contrast with groups, which we discuss in Section 7.1, we will see in Section 7.2 that most of the baryonic (‘normal’) matter in galaxy clusters is not in the galaxies themselves.