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

3. MERGERS AND TIDAL STRIPPING: THE EVOLUTION OF cD GALAXIES

The study of cD galaxies began with the work of W.W. Morgan in the 1960s. Matthews et al. (1964) called attention to the existence in some clusters of bright elliptical galaxies surrounded by extensive, amorphous stellar envelopes (Morgan & Lesh 1965). These they called D galaxies. The largest (and thus the most luminous) of these D galaxies, those with an extent 3-4 times larger than the largest lenticulars (S0s) in the cluster, were dubbed supergiant D: cD galaxies. The large sizes and extensive stellar envelopes (~ 100 kpc) ⁽¹⁾ are the only primary attributes of cD galaxies, and they are the proper criteria to use when classifying clusters (Struble & Rood 1982).

These and later studies (see Bahcall's review for a comprehensive list) have called attention to other secondary characteristics that are neither sufficient nor necessary for a galaxy to be considered a cD. For example, cD galaxies usually reside in the cores of rich, regular clusters (Oemler 1974, 1976); however, both they and their more modest counterparts, the D galaxies, are sometimes found in more irregular clusters at local density enhancements. The cD galaxies often have very extensive (~ 1 Mpc) stellar envelopes of low surface brightness (Oemler 1976) and are frequently found to be at the kinematical centers of their clusters (Quintana & Lawrie 1982). They are often very flat galaxies and are aligned to a flattened distribution of cluster members (Sastry 1968, Dressler 1978c, 1981, Carter & Metcalfe 1980, Binggeli 1982); however, as with other giant ellipticals, their flattening is not primarily due to rotation (Faber et al. 1977, Dressler 1979). Some of the extreme examples appear to have low central and average surface brightness and, usually in concordance, large core radii of many kiloparsecs (e.g. A2029: Dressler 1979). A significant fraction (about 25-50%) have multiple nuclei (Rood & Leir 1979, Hoessel 1980, Schneider et al. 1983).

The evolution of cD galaxies is the best place to begin analyzing the influences of cluster environment on galaxy evolution because cD galaxies do not occur in the low-density feld ( 1 galaxy Mpc^-3). The fact that objects with the primary characteristics of cD galaxies have been found in very poor but high-density groups (Morgan et al. 1975, Albert et al. 1977) is a clear indication that local conditions such as density and velocity dispersion, rather than global parameters like cluster richness or size, are key to understanding the formation of these unique systems.

Unfortunately, much of the discussion about how cD galaxies might form is often muddled by a careless application of the original definition of Matthews et al. For example, two quite dissimilar objects like the cD in A2029 (Dressler 1978b, 1979) and NGC 1316, the brightest member of the Fornax cluster [labeled a cD by Schweizer (1980), but only a D galaxy in Matthews et al.], are compared with the models of cD formation. The cD in A2029 is about a factor of 5 more luminous than NGC 1316, and it is more than an order of magnitude more luminous when the extensive envelope, absent in NGC 1316, is included. In this early stage of comparison of observations with models of formation, it is preferable to restrict one's attention to the largest and most luminous examples of the class, where the predictions made by the models should be manifest in the extreme.