Annu. Rev. Astron. Astrophys. 1980. 18: 165-218 Copyright © 1980 by Annual Reviews. All rights reserved

5.4. Clustering of Galaxies

In recent years there have been several studies on the effect of cluster membership on the structure of radio galaxies. There is general agreement that sources inside clusters have more complex structures than those outside (Lari & Perola 1977, Rudnick & Owen 1977, Burns & Owen 1977, Simon 1978, McHardy 1979). The more bent sources have a larger density of galaxies surrounding them (Stocke 1979) and of the 26 narrow-tailed sources catalogued by Valentijn (1979b) all except two occur in rich clusters. Three narrow-tailed sources occur in the Perseus Cluster (Gisler & Miley 1979) and five others in Abell 2256 (Bridle & Fomalont 1976, Bridle et al. 1979b). Both these clusters have large intrinsic X-ray powers and their galaxies have high radial velocity dispersions. Dynamically active clusters containing dense hot gas are therefore good spawning grounds for tailed sources, as would be expected from the radio-trail hypothesis (Section 2.2.1).

Figure 15. Intensity contours of l.4-GHz emission from the cluster Abell 2256 (Westerbork). The crosses mark the positions of associated galaxies. The cluster contains at least 4 and possibly 8 tailed radio galaxies (from Bridle et al. 1979b).

Because of the preferential occurrence of distorted radio galaxy morphologies in rich clusters it has been suggested that quasars with bent structures may be suitable indicators of X-ray clusters at intermediate and high redshifts (Hintzen & Scott 1978).

In Section 2.2.1 we saw that more highly bent radio sources tend to have optically fainter parent galaxies. Another form in which this correlation manifests itself is the fact that radio sources associated with dominant cluster galaxies tend to be only slightly bent, whereas the narrow-tailed sources tend to be associated with nondominant galaxies (Rudnick & Owen 1977, McHardy 1979, Simon 1978, Valentijn 1979b).

If an intergalactic medium plays a role in confining radio sources, one might well expect the physical sizes of sources to be smaller inside clusters, where the medium is probably denser and hotter. Whether cluster membership influences radio-source sizes is still controversial. McHardy (1979) finds that 4C sources in Abell clusters are not significantly larger than field sources. On the other hand Stocke (1979) and Guindon (1979) concluded that source sizes are systematically larger in regions of low galactic density. In both the latter studies the density of galaxies was determined in the vicinity of strong radio galaxies.