Near - infrared imaging and the K − z relation for radio galaxies in the 7 C Redshift Survey

We present K-band imaging of all 49 radio galaxies in the 7C–I and 7C–II regions of the 7C Redshift Survey (7CRS). The low–frequency (151 MHz) selected 7CRS sample contains all sources with flux-densities S151 > 0.5 Jy in three regions of the sky. We combine the K-band magnitudes of the 7CRS radio galaxies with those from the 3CRR, 6CE and 6C⋆ samples to investigate the nature of the relationship between Kmagnitude and redshift and whether there is any dependence upon radio luminosity. We find that radio galaxies appear to belong to a homogeneous population which formed the bulk of their stars at high redshifts (zf > 5) and evolved passively from then until they reach a mean present-day luminosity of 3L⋆. We find a significant difference between the K-magnitudes of the 7CRS and 3CRR radio galaxies with the 7CRS galaxies being ≈ 0.55 mag fainter at all redshifts. The cause of this weak correlation between stellar and radio luminosities probably lies in mutual correlations of these properties with the central black hole mass. We compare the evolution-corrected host luminosities at a constant radio luminosity and find that the typical host luminosity (mass) increases by approximately 1L⋆ from z ∼ 2 to z ∼ 0.5 which, although a much smaller factor than predicted by semi-analytic models of galaxy formation, is in line with results on optically-selected quasars. Our study has therefore revealed that the small dispersion in stellar luminosity of radio galaxies around 3L⋆ includes subtle but significant differences between the host galaxies of extremeand moderate-power radio sources at fixed redshift, and between those of highand low-redshift radio sources at fixed radio luminosity.