3.2 Frequencies of Cluster Types
We have surveyed a sample of ~ 250 clusters with redshifts less than about 0.15 which were observed by the Einstein Observatory (Jones and Forman 1989a). Of these clusters about 185 are detected and 149 are bright enough to "classify". Of the sample which can be classified, 70% have single peaks and 30% have more than one peak (i.e. multiple substructures). Of this 30%, two-thirds are primarily bimodal (show double structures as in Figure 5, although some may have a considerably weaker third peak). One-third of the multiple-peaked systems are complex with more than two structures (as in Figure 6 and 7).
We also can estimate the fraction of clusters that have central dominant galaxies (XD systems) since Jones and Forman (1984) showed that small core radius clusters have central dominant galaxies. From a sample of about 100 clusters which are sufficiently bright to allow the determination of surface brightness profiles, 40% have small core radii with bright galaxies at their centers. However, only 20% are optically classified as Bautz-Morgan types I and I-II. Thus, only a relatively small percentage have a dominant galaxy based on optical observations, while the X-ray observations suggest that twice as many clusters have central dominant galaxies. This difference in the number of clusters with dominant central galaxies arises since the two brightest galaxies in a considerable number of clusters have comparable optical luminosities and, hence, are of late Bautz-Morgan type. However, only one of the galaxies is centrally located in the cluster (e.g., M87 in Virgo and N708 in A262).