Annu. Rev. Astron. Astrophys. 1982. 20: 547-85 Copyright © 1982 by Annual Reviews. All rights reserved

4.3 Spiral-Rich Clusters with cD Galaxies

Abell 400, Abell 1991, Abell 2063, and Abell 2199 are examples of spiral-rich clusters with dominant galaxies whose X-ray gas is predominantly cool (Mushotzky & Smith 1980 and this paper). Abell 2634 is a possible member of this class based on the presence of a central, optically dominant galaxy, although the cluster has a larger core radius than other XD clusters (Jones et al. 1979). In these and more evolved XD clusters, the X-ray surface brightness profile is a single smooth curve. Therefore one cannot distinguish an end to the galaxy mass distribution or a beginning to that of the cluster. These clusters have X-ray core radii of 300 kpc, X-ray luminosities between 2 x 10⁴³ and 2 x 10⁴⁴ erg s^-1 in the 0.5-3.0 keV band, and central gas densities of ~ 10^-2-10^-3 cm^-3. Optically, these five clusters display a range of Bautz-Morgan classes from I to II-III (Leir & van den Bergh 1977) and Rood-Sastry types from cD to F and I (Rood & Sastry 1971). Although the optical classifications are mixed, the X-ray images (see Figure 8) demonstrate that each cluster is of the XD type with peaked X-ray emission centered on a bright galaxy (see also Jones et al. 1979 for surface brightness profiles and the X-ray image of Abell 2634). These rich clusters differ from those typified by Abell 85 (Section 4.4) both in their lower X-ray temperatures and luminosities and optically in their high spiral fractions [ 40% for Abell 400, Abell 1991, Abell 2063, and Abell 2634 - Dressler 1980b; for the central region of Abell 400, Oemler (1974) determined a galaxy content of 15% E, 56% S0, and 29% S]. They also possibly differ by the presence of multiple components in the central galaxy. Although the presence of a cD galaxy with multiple nuclei indicates that galaxy mergers have occurred, other dynamical indicators suggest that these systems are relatively unevolved.

Figure 8. The 0.5-3.0 keV smoothed IPC images of Abell 400 (upper left), Abell 1991 (upper right), Abell 2063 lower left), and Abell 2199 (lower right). The contour levels correspond to 3, 5, 7, 9, 11, and 15 sigma, 3, 5, 7, 15, 25, and 35 sigma, 3, 7, 11, 15, 19, 23, 27, and 31 sigma, and 3, 5, 7, 9, 13, 17, 25, 35, 45, 60, and 70 sigma for Abell 400, Abell 1991, Abell 2063, and Abell 2199 respectively.