Annu. Rev. Astron. Astrophys. 1984. 22: 185-222
Copyright © 1984 by . All rights reserved

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A small fraction (about 5%) of the galaxies in the present-epoch Universe are collected into groups and clusters whose space density is larger than one galaxy per cubic megaparsec ( about two orders of magnitude greater than the average). Some of these aggregations are both dense and populous. These are the rich clusters cataloged by Abell (1958) during the original Palomar Sky Survey. Typically they contain 100 galaxies with a luminosity spread of two orders of magnitude (and perhaps a larger number of fainter systems) in a volume several megaparsecs across. A roughly equal number of galaxies inhabit relatively poor groups with less than 30 galaxies, which also have relatively high densities (Bahcall 1980).

The Virgo cluster, the rich cluster nearest to our Galaxy, is a representative example, with the typical number and density of galaxies. Its structure is also quite ordinary, for although it has the beginnings of a well-developed core, a general lack of symmetry and significant subclustering indicates that dynamical relaxation on a large scale has just begun. Its population of galaxies is normal, with all morphological types [ellipticals, S0s, spirals (and irregulars)] more or less equally represented. Thus, in all these characteristics measured by optical techniques, the Virgo cluster is a very average cluster, as one might expect of our nearest neighbor.

Perhaps it has been a mixed blessing that another of the nearest clusters, the Coma cluster, is a very rare specimen. Its proximity has allowed detailed studies of a cluster that is unusually populous and rich in early-type galaxies, but it has also contributed to a widespread notion that Coma is a typical rich cluster. In fact, it is no more typical of rich clusters than M87 is typical of galaxies. The Coma cluster, with its population of several hundred bright galaxies, is richer than 95% of the clusters catalogued by Abell. Its galaxy distribution is highly concentrated and very symmetric, with little or no sign of subclustering; this probably indicates that it virialized many cluster crossing times ago [several x 109 yr (Gunn & Gott 1972)]. The population of its core is extreme, as it contains nearly equal numbers of elliptical and S0 galaxies and virtually no spirals or irregulars. The great advantage in having a cluster like Coma so nearby is that one can study an extreme environment where galaxy evolution may have been most influenced by the formation and evolution of the cluster. To the extent that the formation and evolution is a function of the local environment (e.g. the local density and characteristic velocity of the galaxies), the processes that have influenced galaxy development in the Coma cluster are a smooth extrapolation of what has happened in the less populous but more numerous groups of galaxies.

It is the long-term goal of the studies reviewed here to understand how galaxy-galaxy and galaxy-cluster interactions have influenced the development of different morphological types, the nuclear and star-forming activity of galaxies, and the structure and luminosity function of member galaxies. The processes that have been proposed are reviewed here and compared with observational data, chiefly from optical measurements. Most of these processes depend on one or more of the following parameters: (a) the local density of galaxies and/or gas, (b) the kinematics of the galaxy distribution, and (c) the types, structures and internal kinematics of the galaxies present. Given the wide range of environments, from the low-density field to the richest clusters, it should be possible to evaluate the domain of importance for each model and thus arrive at a better understanding of how galaxies form and evolve.

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