Low-power radio galaxy environments in the Subaru?/XMM-Newton Deep Field at z ~ 0.5

Abstract

We present multiobject spectroscopy of galaxies in the immediate (Mpc-scale) environments of four low-power (L_1.4GHz <~ 10²⁵ WHz^-1) radio galaxies at z ~ 0.5, selected from the Subaru?/XMM-Newton Deep Field. We use the spectra to calculate velocity dispersions and central redshifts of the groups the radio galaxies inhabit, and combined with XMM-Newton (0.3-10keV) X-ray observations investigate the L_X-σ_v and T_X-σ_v scaling relationships. All the radio galaxies reside in moderately rich groups - intermediate environments between poor groups and rich clusters, with remarkably similar X-ray properties. We concentrate our discussion on our best statistical example that we interpret as a low-power (Fanaroff-Riley type I) source triggered within a subgroup, which in turn is interacting with a nearby group of galaxies, containing the bulk of the X-ray emission for the system - a basic scenario which can be compared to more powerful radio sources at both high (z > 4) and low (z < 0.1) redshifts. This suggests that galaxy-galaxy interactions triggered by group mergers may play an important role in the life-cycle of radio galaxies at all epochs and luminosities.