Annu. Rev. Astron. Astrophys. 1994. 32: 115-52 Copyright © 1994 by Annual Reviews. All rights reserved

The Effect of Environment

Perhaps the strongest differences among galaxies of different morphologies are seen in their clustering tendencies. In individual cases, the importance of mergers, tidal interactions and sweeping within clusters can readily be demonstrated, yet the overall imprint of recent processes on galaxy morphology remains unclear. In this section, we review the evidence for morphological segregation and the importance of interactions between galaxies and their surroundings.

MORPHOLOGICAL SEGREGATION As early as the 1880's, Wolf noticed that the distribution of nebulae was not uniform in the sense that more elliptical nebulae were concentrated in the Virgo direction than elsewhere. By the 1930's, the morphological differences between field and cluster galaxies were well established (Hubble and Humason 1931). Using his survey of 55 rich clusters, Dressler (1980) has quantified the concept of morphological segregation, showing the steady decrease in spiral fraction and the corresponding increase in the E/S0 population with local galaxy density, a variation in population fraction that is slow but monotonic. Extending Dressler's study, Postman and Geller (1984) have shown that the morphology-density relation holds over six orders of magnitude in space density to regimes where the dynamical timescale approaches the Hubble time.

THE CLUSTER ENVIRONMENT In the highest density environments, the possible morphology altering mechanisms are many; galaxy-galaxy, galaxy-cluster and galaxy-intracluster medium interactions can all lead to significant changes in morphology and star formation potential. Indeed, the occurrence of pathological and disturbed objects in high density regions is well-recognized. Dressler (1984) reviews the models for morphological alteration in clusters according to the relative importance of initial conditions or late evolution. Whitmore (1990) gives a recent summary of the various galaxy properties that are seen to vary significantly between cluster and field galaxies.

In addition to the obvious variation in morphological make-up, various authors have attempted to identify density dependences in the fundamental properties under consideration in this review. It has already been mentioned in Section 3 that spiral galaxies passing through the center of rich X-ray clusters appear to lose up to 90% of their interstellar HI. At the same time, their molecular consitutents, as measured by their CO content and distribution, remain relatively unaffected (Kenney & Young 1989). Evidence for the stripping of spirals in clusters and constraints on the responsible processes are reviewed in Haynes (1990).

Most recently, studies have addressed the possibility of environmental variations in the distribution of mass within galaxies, and the results are conflicting. The detailed studies of rotation curves of spiral galaxies in clusters by Rubin et al. (1988) and Whitmore et al. (1988) suggest that, in inner cluster members, the halo is either partially stripped or not allowed to form, a conclusion based on the observation of falling rotation curves in centrally located galaxies. However, Distefano et al. (1990), using H rotation curves, and Guhathakurta et al. (1988), using ones derived from HI synthesis maps, do not see such environmental effects in their respective studies of Virgo members.

THE GROUP ENVIRONMENT In loose groups, where the velocity dispersion is low, slow close prograde tidal encounters can remove significant fractions of a galaxy's interstellar material. A classic, graphical discussion of the tidal phenomenon is given by Toomre and Toomre (1972), and numerous examples of the success of these models in reproducing tidal bridges and tails are now available. Both radio emission and far-infrared emission are strongly enhanced in the instance of tidal interactions. The importance of interactions in a wide range of phenomena from the formation of shells and polar rings to the driving of spiral stucture and starburst phenomena have been discussed by numerous authors, most recently Barnes & Hernquist (1992).

GALAXIES AT HIGH REDSHIFT Evidence is now accumulating that significant evolution of the cluster population has occured between the present time and the epoch corresponding to a redshift z 0.4. Clusters in the range 0.4 z 1 show a higher fraction of blue galaxies than do their low redshift counterparts, the so-called Butcher-Oemler effect. Gunn (1990) reviews the evidence for the Butcher-Oemler effect including the increase in emission-line and Seyfert objects and the presence of the ``E + A'' population. Recent high resolution imaging of the blue cluster members confirms their spiral nature (Lavery et al. 1992; Dressler & Gunn 1992). The relationship of the present day population to the distant cluster galaxies and their field counterparts is critical to our understanding of the process of galaxy evolution and the development of the morphological characteristics evident today.