THE EVOLUTION OF GALAXIES IN RADIO-SELECTED GROUPS

In order to study the evolution of galaxies in small groups in a way unbiased by optical selection effects, we have observed the populations of galaxies around a sample of 98 strong radio galaxies, 65 with redshifts less than 0.25, and 33 with redshifts between 0.25 and 0.50. These radio galaxies were chosen to have the same range of radio power 10(26) < p408 < 10(28) W Hz-1, at both high and low redshift. CCD photometry of fields containing the radio sources, and of nearby comparison fields, was obtained in the B and V bands for the low-redshift sample, and in the V and R bands for the high-redshift sample. We have analyzed the properties of galaxies with M(V) less-than-or-equal-to -19.0 (assuming H-0 = 50 km s-1 Mpc-1 and q0 = 0.0) within 0.5 Mpc of the radio galaxies, with the following results. (1) Strong radio galaxies occur in groups whose range of richness is broad, but not as broad as that of groups in general: radio galaxies may tend to avoid very poor groups. (2) There has been a slight decrease in the richness of radio groups since the epoch observed at z = 0.4, particularly among the FR II sources. (3) The luminosity function of the members of both high- and low-redshift radio groups is indistinguishable from that of nearby field galaxies. (4) Groups around nearby radio galaxies display the same relation between population and environment as do groups in general: richer groups contain redder galaxies. (5) However, groups at high redshifts show little indication of such a trend: groups of all richnesses have similar high fractions of blue galaxies. We conclude that galaxies in groups containing radio galaxies are typical of group members in general and are a good probe of the properties of normal galaxies. The difference between the properties of our low- and high-redshift samples suggests that the morphology-environment relation is of recent origin and is steepening rapidly with time. This steepening is most likely responsible for the rapid evolution previously observed in the colors of galaxies in rich clusters.