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

We present multiobject spectroscopy of galaxies in the immediate (Mpc-scale) environments of four low-power (L1.4 GHz≲ 1025 W Hz−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 (0.3–10 keV) X-ray observations investigate the LX–σv and TX–σv 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.