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

1. INTRODUCTION

Even though they contain only a small fraction of the galaxies in space, rich clusters stand out distinctly against the fabric of galaxies that make up the visible Universe. Because of their high surface densities and large number of very luminous galaxies, clusters can be identified out to distances comparable to the present horizon of the Universe, making them important tools in the study of cosmology (Hubble 1936, Zwicky 1938). Because of this legacy, research on clusters of galaxies has traditionally centered on the measurements of standard candles (e.g. the luminosity of the brightest cluster member, or the ``knee'' in the cluster luminosity function) or standard metrics (radii of cluster galaxies or the cluster distribution as a whole). The review article by Bahcall (1977) reflects a goal of researchers at that time: to provide a morphological description of the sizes, shapes, and galaxy content of clusters, and whenever possible, the applicability of these parameters to cosmological investigations.

In the 1970s a new course of research on clusters of galaxies began to emerge with the resurgence of interest in galaxy evolution. It was recognized that certain clusters contain ``supergalaxies'' unlike any objects seen in the general field. How had these objects evolved in the unique environment of clusters? The general population of galaxies in clusters is highly skewed toward elliptical and S0 galaxies, a population quite unlike the spiral-dominated field where most galaxies are actively forming new stars. What could this obvious difference tell us about the influences of environment both in the formation and evolution of the different morphological types? It was discovered that many rich clusters contain a pervasive, hot intergalactic gas. Might interactions between this gas and galaxies be strong enough to alter galaxy properties such as their own gas fraction and rates of star formation? Clusters, it was realized, are laboratories for the study of galaxy evolution and may become as useful as star clusters are in the study of stellar evolution.

This review is restricted to recent work on clusters and groups of galaxies that is relevant to the study of galaxy evolution. Such subjects as cluster morphology, structure (e.g. spatial distribution of galaxies, core radii), luminosity functions, mass-to-light ratios, catalogs, and dynamics are discussed only in connection with how galaxies may form and change in response to the cluster environment. Also beyond the scope of this review is the rapidly growing area of research that uses large-scale clustering properties to model the evolution of the early Universe (e.g. the distribution of clusters in space, the multiplicity function for clusters, the evolution of clusters and superclusters from primordial fluctuations). Two other areas have grown so rapidly since the Bahcall review that they warrant their own chapters: X-ray emission from clusters (Forman & Jones 1982) and the HI content of cluster galaxies (Haynes et al. 1984). These reviews are themselves highly relevant to the topics discussed here.

The following discussion is divided into four sections: the laboratory - a brief introduction to clusters of galaxies; mergers, tidal stripping, and accretion - the evolution of cD galaxies; the development of the morphological types; and the populations of clusters as a function of cosmological look-back time.