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

4. CLUSTERS WITH DOMINANT, CENTRAL GALAXIES - XD CLUSTERS

By combining X-ray and optical observations, we now discuss the "cD phenomenon" in X-ray clusters and groups. We begin with the general properties and origin of dominant cluster galaxies (Section 4.1). Next we review the Virgo cluster (Section 4.2), which contains an unusually massive galaxy (M87), and whose average properties indicate that it is relatively unevolved. Other spiral-rich clusters that contain X-ray-dominant galaxies are discussed in Section 4.3. These clusters are notable for their low X-ray gas temperatures (~ 2 keV), which imply a shallow cluster potential. Section 4.4 summarizes the properties of spiral-poor clusters with centrally located, dominant galaxies (e.g. Abell 85 and Abell 1795). Observations of the Perseus cluster are presented in Section 4.5, and we discuss in Section 4.6 the X-ray emission from groups of galaxies that should be dynamically evolved because of their short dynamical timescales. All these systems are members of the XD cluster family, which is characterized by X-ray emission with a core radius of 300 kpc centered on a central, dominant galaxy.