Annu. Rev. Astron. Astrophys. 1984. 22: 445-70 Copyright © 1984 by Annual Reviews. All rights reserved

3.1 The Group Environment

Groups of galaxies come in a wide spectrum of sizes and membership; basically, a group is often defined as a density enhancement of about 10² to 10³ with respect to its surroundings. Groups can be further categorized as loose or compact, with the compact structures possibly only temporarily unbound subcondensations existing within more widespread structures (91a). In comparison to the richer clusters, groups have a higher proportion of spiral galaxies; only a very few of the groups in de Vaucouleurs' (29) list are dominated by galaxies earlier than Sa. Velocity dispersions within groups are on the order of a few hundred kilometers per second, and in a few interesting instances, such as the IC 698 group (118), they are much smaller. X-ray emission has been found in prominent galaxies in two nearby loose groups, NGC 3607 (8) and NGC 5846 (7), with X-ray luminosities that follow an extrapolation of the relationship between X-ray luminosity and velocity dispersion found for clusters. Both of these groups, however, are distinctive among other nearby loose groups in having higher velocity dispersion and a larger representation of early-type objects. In general, the density of the intergalactic gas in groups is low, and the major influence of group membership is simply the presence of nearby neighbors.

In both clusters and groups, the increased density of the galaxy population enhances the likelihood of a tidal interaction among members. The number of encounters is expected to be highest in clusters, where the space density is large. But the potential for actual damage done to an individual galaxy, the ``disruption damage'' as discussed by Cottrell (26), depends on the degree of penetration, the duration of the encounter, and the sense of the orbital and rotational motions. In the impulsive approximation, the disruption damage goes as 1/v_r, where v_r is the relative velocity of the galaxies involved. Significant perturbations of the disk occur only when the relative velocities are close to the parabolic (108). Tidal encounters exert strong influences on group galaxies that stray too close to one another. However, because the relative velocities expected in clusters are much higher than in groups, a typical collision in a rich cluster will render only about one tenth the damage as one with the same impact parameter and mass ratio that takes place in a small group.