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Session 116 - Galaxy Clusters & Evolution.
Oral session, Saturday, January 10
Lincoln,
Results of a study of the radio galaxy population in 54 very-rich, Abell clusters, from 0.02 \le z \le 0.41, indicates that the number of cluster galaxies with enhanced radio emission increases with redshift. We have obtained deep, wide-field 20 cm VLA observations, B-band and R-band wide-field CCD images, and follow-up spectroscopy on most cluster sources with optical/radio identifications. Four redshift samples were constructed, with completeness in the radio to luminosities of 10^22 W Hz^-1 (H_0=75) and in the optical to M_R \le-21^m. Each of the 14 low-redshift clusters (0.10 \le z \le 0.18) generally contain fewer than 10 galaxies with radio emission above our limit. Of the 12 intermediate redshift (0.20 \le z \le 0.25) clusters, A2125 is the most extreme. This Butcher-Oelmer (BO) cluster has at least 30 cluster member with enhanced radio emission. In our most distant sample (0.38 \le z \le 0.41) of 4 BO clusters, cl0024+16 has almost double the cluster members with enhanced radio emission as A2125. Spectroscopic classification of these radio galaxies has shown them to be a mixture of both AGNs and star-bursting galaxies. Since the four redshift samples are comprised of similar richness clusters this effect is not a function of cluster richness. Rather, a function of the cluster's redshift. Therefore, our results suggest that the cluster population has undergone an enhancement in its radio emission and that the radio galaxy population in very-rich clusters has significantly evolved over the last 5 Gyr.
