The Megaparsec Environments of Radio Galaxies

Abstract

This paper presents an analysis of the Mpc environments of powerful radio galaxies (with z < 0.5 and radio powers P408 MHz > 1026 W Hz-1, using H0 = 50 km s-1 Mpc-1 and q0 = 0.0). We find that most radio galaxies reside in rather poor clusters (or loosely "groups") that have three to 10 members (whose magnitudes are brighter than -19 and that lie within 0.5 Mpc of the radio galaxy). Although there is a possibility that up to 5% of all radio galaxies are field galaxies (and up to 13% are pairs), our result is consistent with all radio galaxies being in groups. The distributions in group richness and Bautz-Morgan (BM) class of radio-selected groups are different from those of optically selected groups. Radio-selected groups are preferentially of BM type I, while optically selected groups are preferentially of BM type III. The richness distributions are comparable for rich groups (with N0.5−19>12); however, poor radio-selected groups are ~1.5 times less abundant.