The black holes of radio galaxies during the Quasar Era: Masses, accretion rates, and evolutionary stage

Abstract

We present an analysis of the AGN broad-line regions of 6 powerful radio galaxies at z ∼ 2 (HzRGs), which is part of a study of a sample of 50 HzRGs with rest-frame optical imaging spectroscopy obtained at the VLT. In 6 galaxies we detect luminous (L(Ha) = few × 1044 erg s-1), spatially unresolved, broad (FWHM = 10 000 km s-1) Ha line emission from the nucleus (HaBLRs), consistent with broad-line regions of supermassive black holes with masses of few × 109 M and accretion luminosities of a few percent of the Eddington luminosity. In two galaxies we also detect HßBLRs, suggesting relatively low extinction of AV ∼ 1 mag, which agrees with constraints from X-ray observations. Overall, we find HaBLRs in ∼20% of the galaxies where Ha is observed. By relating black hole and bulge mass, we find a possible offset towards higher black-hole masses of at most ∼0.6 dex relative to nearby galaxies at a given host mass, although each individual galaxy is within the scatter of the local relationship. If not entirely from systematic effects, this would then suggest that the masses of the host galaxies have increased by at most a factor ≈4 since z ∼ 2 relative to the black-hole masses, perhaps through accretion of satellite galaxies or because of a time lag between star formation in the host galaxy and AGN fueling. We also compare the radiative and mechanical energy output (from jets) of our targets with predictions of recent models of "synthesis" or "grand unified" AGN feedback, which postulate that AGN with similar radiative and mechanical energy output rates to those found in our HzRGs may be nearing the end of their period of active growth. We discuss evidence that they may reach this stage at the same time as their host galaxies. © ESO 2010.