4.5. Individual clusters
Coma is the prototype of the nXD regular cluster, and is often used as a general prototype for comparing models for relaxed clusters. Its optical image is given in Figure 1b. It is worthwhile noting that clusters as rich and compact as Coma are actually rather uncommon. Coma is also unusual in having the most prominent radio halo observed among clusters (Section 3.4). Coma and A1367 (see below) appear to be portions of a large supercluster system (Gregory and Thompson, 1978).
The X-ray image of Coma, which is shown in Figure 19, shows that the X-ray emission is somewhat elongated (Johnson et al., 1979; Gorenstein et al., 1979; Helfand et al., 1980) in the same direction as the galaxies in the cluster (Section 2.7). The X-ray emission shows a central uniform core (Helfand et al., 1980; Abramopoulos et al., 1981) and falls off with radius more slowly than the galaxy distribution ( 0.8 in equation 4.7). Coma has an unusually high X-ray temperature for its galaxy velocity dispersion (Mushotzky et al., 1978), although this is consistent with its small value for . The X-ray surface brightness from Coma is very smooth, and there is no apparent excess emission associated with the two central galaxies (Forman and Jones, 1982; Bechtold et al., 1983).