Annu. Rev. Astron. Astrophys. 1979. 17: 135-87 Copyright © 1979 by Annual Reviews. All rights reserved

6. DYNAMICS OF SMALL GROUPS OF GALAXIES

Over the past decade, our knowledge of the statistics of galaxy clustering has increased enormously, due in large part to the pioneering analysis of galaxy positions by Peebles and co-workers [see Proceedings of I.A.U. Symposium No. 79, The Large Scale Structure of the Universe (Longair & Einasto 1978) for discussions and references to earlier work]. The great majority of galaxies are apparently located in groups (Gott & Turner 1977, Soneira & Peebles 1977, Gregory & Thompson 1978). The definitive determination of group masses would therefore yield a representative estimate of the mass associated with galaxies in the universe as a whole.

Studies of galaxy clustering have demonstrated quite clearly that clusters exist on all scale sizes from the traditional great clusters like Coma down to binaries. Dynamically speaking, then, there exists a continuum, and there is no obvious dividing line between clusters and smaller groups. In practice, however, analyses of great clusters and small groups are unlike in a number of ways. Because the membership sample in groups is usually very small, the procedures for statistical mass determination and error estimate differ substantially from those in great clusters, where techniques based on large samples are more appropriate. Furthermore, the contrast of the cores of rich clusters against the background is much larger than for small groups, and the treatment of foreground-background contamination is therefore different. Finally, there are a number of dynamical processes more likely to occur in large clusters, where the densities are large and the crossing times short. Foremost among these are dynamical friction and tidal stripping, both of which can significantly rearrange the distribution of matter inside the cluster. For these reasons, we reserve the discussion of large clusters to Section 7, and treat here only the analysis of small groups.

Earlier work on groups of galaxies culminated in de Vaucouleurs' (1975) monumental listing of nearby associations, a systematic survey of volume density enhancements within 35 Mpc. Membership determination was based on absolute magnitude indicators as well as on redshifts. Sandage & Tammann (1975) presented a catalog selected in similar fashion. Despite the use of all available criteria in addition to redshift, the assignment of galaxies to groups remained a very difficult question and one without any obvious statistical solution. A major new development has been the identification of groups via a surface-density criterion only (Turner & Gott 1976). In this technique, one accepts the inevitable contamination by foreground and background galaxies as the price one must pay for a statistically well-defined and unbiased sample. However, one then must find a satisfactory method for dealing with the contamination problem.