3.3.4. cD Galaxies. I. Central Structure

In section 2.2 I reviewed the definition of cD galaxies as supergiant elliptical-like galaxies with very extensive outer halos. The classic and most extreme examples are brightest galaxies in rich clusters. They are clearly distinguishable from ordinary ellipticals both because they are so much brighter (Oemler 1976; Dressler 1978, 1979; Thuan and Romanishin 1981) and because their halos are enormous compared to halos of ellipticals (e.g., Morgan and Lesh 1965; Oemler 1973). However, the transition to ordinary ellipticals is smooth. Therefore it is not immediately clear how we should classify "cD galaxies in poor clusters" (Morgan, Kayser and White 1975; Albert, White and Morgan 1977), the brightest galaxies NGC 4874, 4889 and 4839 in the Coma cluster, or even M87. What are the quantitative properties of cD galaxies, and to what extent do they imply physical differences from ordinary ellipticals?

The central parts of cD galaxies are superficially similar to very bright ellipticals. Their profiles satisfy r^1/4 laws (Thuan and Romanishin 1981) or Hubble laws (Oemler 1973, 1976). Photometric parameters of cD galaxies in rich clusters are also normal: they fall on the extrapolation toward higher luminosity of the B_e - logr_e relation for ordinary ellipticals (Fig. 13). The same conclusion has been reached by Hoessel (1980, see Fig. 17) from his photometry of 108 first-ranked cluster ellipticals, many of which are cDs. Thuan and Romanishin (1981, Fig. 10) find that cD galaxies in poor clusters also fall on the brightward extrapolation of the B_e - logr_e relation. In fact, their Hubble-law parts have the same luminosities as corresponding parts of cDs in rich clusters. The "reduced magnitudes" M_red = - 2.5 log(I₀ a²) + constant for nine cDs in rich clusters (Oemler 1976) have an average value of <M_red> = -20.58 (dispersion = 0.40). Nine cDs in poor clusters have <M_red> = -20.69 (dispersion = 0.56). Thus the main bodies of cD galaxies in rich and poor clusters are photometrically indistinguishable. Both are indistinguishable from ordinary ellipticals except for the fact that on average they are more luminous. cD Galaxies extend the magnitude range of the B_e - log r_e relation for ellipticals by about 2 mag to -19 M_B -25 (H₀ = 50 km s^-1 Mpc^-1).

One very important observation distinguishes first-ranked cluster ellipticals, including cDs, from ordinary ellipticals. Close doubles and multiple nuclei are surprisingly common considering their short lifetimes before they merge into one object (Matthews, Morgan and Schmidt 1964; Morgan and Lesh 1965; Jenner 1974). The best-known example is NGC 6166, which has four nuclei very close to each other in projection deep inside the light distribution (Minkowski 1961). Another impressive object which cannot long survive unchanged is V Zw 311, which has at least nine components imbedded in a common halo (Zwicky 1971, p. 102; Gunn 1977b). Hoessel (1980) has shown that ~ 28% of first-ranked cluster galaxies have multiple nuclei. These observations constitute the best evidence that galaxy mergers play an important role in the formation of cD and other first-ranked galaxies (e.g., Ostriker and Tremaine 1975; see Tremaine 1981 and White 1982 for reviews). If this is so, then the similarity of cD and elliptical galaxies implies either that ordinary ellipticals also form through mergers, or that merger remnants are photometrically indistinguishable from other ellipticals.