Perhaps the three most basic questions an extragalactic astronomer might be asked are:
Although we can fill journals with details about galaxies, an astronomer cannot really answer these three basic questions with any confidence. The first question raises the "nature vs. nurture" problem. Is the morphological type of a galaxy determined at birth by its initial conditions, or does the environment of the galaxy control its destiny? While the standard picture that different initial conditions cause the difference in morphology still seems plausible (e.g., Sandage, Freeman, and Stokes 1970, Gott and Thuan 1976), recent interest has centered on Toomre's (1977) suggestion that all galaxies begin as disks and then merge to form elliptical galaxies. The problem with the second question is that a galaxy does not really have an "edge", so the definition of size is somewhat arbitrary. What makes things worse is that rotation curves in spiral galaxies stay flat as far out as we can observe them, so the mass is still increasing linearly with radius with no sign of the edge of the dark matter in sight. The third question is also impossible to answer very precisely since we don't know what or where the dark matter is in a galaxy. The fact that only about 10% of a galaxy is luminous means that we are studying only the tip of the iceberg, and shows just how little we really know about galaxies.
One of the principle reasons many of us are interested in the study of clusters is that these three basic properties of a galaxy, the morphological type, the size, and the distribution of mass, all appear to be affected by the cluster environment. As early as 1931 (Hubble and Humason 1931) it was known that elliptical galaxies were preferentially found in clusters, providing evidence that the morphological types were closely linked with the environment. The cluster environment also appears to be able to modify the size of a galaxy. cD galaxies can grow to prodigious size, possibly at the expense of the nearby galaxies which become shrunken via tidal stripping. Recent measurements of emission-line rotation curves also suggest that the massive halos of spiral galaxies near the centers of clusters may be stripped, or perhaps were never allowed to form in the first place. The fact that these three basic galactic properties all vary as a function of position within a cluster should provide a fundamental clue about the formation of galaxies. If we are going to answer the three questions posed above in the near future it is likely to be from studies of galaxies within clusters.
This article will not be a comprehensive review of the subject, but instead will focus on some of the key studies which are relevant to the three questions posed above. The reader is referred to recent review articles by Haynes, Giovanelli, and Chincarini (1984) and Haynes (1990; radio observations), Richstone (1990; cluster simulations), Binggeli, Sandage, and Tammann (1988; luminosity functions), White (1982; theoretical mechanisms) and Dressler (1984a; general review), for more comprehensive reviews.
Our approach will be to examine the evidence for variations in the morphological type, size, and mass as a function of environment; from the dense inner regions of rich clusters and compact groups, to the sparser regions in the outer parts of clusters, the loose groups, and the field. The emphasis will be on methods to distinguish which of the proposed mechanisms are responsible for the various effects.
This review will concern itself primarily with optical observations. However, various theoretical mechanisms which may be responsible for modifying the properties of galaxies will be briefly discussed in Section 2, in order to develop a framework from which to examine the observations. Sections 3, 4, and 5 will then examine how the morphological type, size, and distribution of mass within a galaxy are affected by its environment. A short summary is provided in Section 6.