The cluster radio source population

Many clusters of galaxies contain radio sources, and many radio sources are in galaxy clusters. For example, the Coma cluster contains a number of radio sources (Willson 1970; Kim et al. 1990; Venturi, Giovannini & Feretti 1990). The cluster A2256 contains several tailed sources (Bridle & Fomalont 1976). The powerful radio galaxies Cygnus A and 3C295 are both in X-ray clusters (Arnaud et al. 1984; Henry & Henriksen 1986).

Studies of powerful radio galaxies have shown that, at intermediate redshifts, radio sources tend to be in cluster environments (Hill & Lilly 1991), a trend which continues as one goes to higher redshifts (Hintzen, Ulvestad & Owen 1983; Yee & Green 1987; Yates, Miller & Peacock 1989).

Cross-correlations of radio catalogs with cluster catalogs such as that of Abell (1958) have shown that there is a population of radio sources associated with galaxy clusters. McHardy (1979) found that about 20% of radio sources in the luminosity range P_408=10^25--10^26.5 W/Hz were in Abell R>1 clusters. For the Molonglo survey of the southern sky, Robertson & Roach (1990) found an excess of associations with galaxy clusters, including a narrow peak at small radii that is probably due to the central cluster galaxy being a radio source.

In addition to sources in clusters, we should also include those sources in smaller structures that will later be incorporated into clusters. This includes small groups and clusters not rich enough to be included in the Abell catalog. Burns & Owen (1979) studied 4C sources in Zwicky clusters and concluded that most if not all intermediate luminosity radio galaxies are in clusters of various richnesses.

Thus, a typical cluster will have been host to several radio sources in its lifetime. The subclusters that went into making a rich cluster also contained radio sources. The majority of radio sources were either in clusters or in units that were later incorporated into clusters, so that most of the radio sources that have existed thorughout the history of the universe have since been incorporated into clusters, and those presently not in clusters will be incorporated into clusters at some future time.

Peter Tribble,