Cluster Membership

In a first attempt at separating cluster members from interlopers in our sample, we have made use of the data compiled by Auriemma et al. (1977) on the local radio luminosity function of elliptical galaxies. By combining radio data on galaxies from four different published lists, they showed that galaxies having radio emission above detection limits similar to ours span absolute photographic (nearly equivalent to blue) magnitudes from -18.5 to -22.5 for our adopted . Absolute magnitudes were calculated for all of our identified radio galaxies, assuming they were at the cluster distance. Those falling outside the above range are probably interlopers (mostly background galaxies), while those within this range are likely to be cluster members.

We show in Figure 4 the projected spatial distribution of radio galaxies with magnitudes falling in the above range for the 23 clusters of distance class D 4 (); the distance class restriction allows us to consider a complete sample. As in previous studies (eg. Mills and Hoskins, 1977; Robertson and Roach, 1990; Unewisse, 1993; Ledlow and Owen, 1995a), we find a large excess of radio sources towards the centres of the clusters, and a low but uniform spread in the outer regions, suggesting that non-cluster members may be contaminating the distribution. Although refinement of the separation process will be necessary, the background is considerably reduced compared with previous work.

An interesting result from Figure 4 is the excess over the background for projected distances between 100 and 500 kpc from the cluster centre. A similar feature was reported by Robertson and Roach (1990) but is seen here with greater contrast because of the successful removal of most of the background population, together with a more thorough identification procedure, especially for extended sources. Since the radial averaging in Fig. 4 acts to smear out the radio galaxy distribution, our follow-up analysis of this sample will use the COSMOS database to examine more closely the radio detection rate as a function of local galaxy density.

© Copyright Astronomical Society of Australia 1997