Clusters of galaxies

Abstract

Available data on clusters of galaxies are reviewed. The main conclusions are the following: 1. (1)|The Virgo Cluster has a complex structure. A core, in which m L ⋍ 500 m ⊚ L ⊚ , is surrounded by clouds of spiral galaxies. 2. (2)|M87 is located close to the cluster core and may be almost stationary relative to it. This may have resulted in inflow of intra-cluster gas which prolonged the halo phase of the collapse of this galaxy thus accounting for the unusually large number of globular clusters per unit mass that this galaxy contains. 3. (3)|Little or no correlation is found between the BM classification of clusters and their radio emission. Clusters in which the first-ranked galaxy is close to the cluster centre are, however, more likely to emit radio radiation than is the case for clusters that do not contain first-ranked galaxies near their centres. This result could be understood if first-ranked central galaxies have little or no motion relative to the intra-cluster gas so that interstellar gas is not removed from them by rampressure. 4. (4)|A survey of radio radiation from 503 Abell clusters shows that rich clusters do not have a significantly higher probability of being radio sources than do poor clusters. It follows that individual bright galaxies in poor clusters have a higher probability of becoming radio sources than do those in rich clusters. This result may be understood if it is assumed that: 4.1. (i)|Radio explosions are produced by gas that was initially ejected by evolving stars, and 4.2. (ii)|Rampressure removes this gas most efficiently from rich clusters in which the velocity dispersion is large. 5. (5)|The X-ray luminosity of clusters does not depend on the position of the first-ranked galaxy. X-ray clusters do, however, tend to be richer than average and frequently contain dominant cD galaxies. 6. (6)|Observations of morphological peculiarities of cluster spirals, the frequency of S0 galaxies, the existence of radio tails and thermal bremsstrahlung are all consistent with the presence of an intra-cluster medium with T ∼ 10 8 K and p ∼ 10 −27 gm cm −3. 7. (7)|The observed distribution of axial ratios of first-ranked galaxies in Abell clusters is indistinguishable from that of nearby ellipticals of lesser luminosity. This result is difficult to reconcile with models in which galaxies derive their angular momentum from tidal torques. 8. (8)|First-ranked galaxies in clusters of Bautz-Morgan type I are, on the average, more highly flattened than are those in dynamically less-evolved types of clusters. This suggests that mergers resulting from dynamical friction might account for some of the most highly flattened cD galaxies in clusters of BM class I.