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

TL;DR

This study analyzes supermassive black holes in high-redshift radio galaxies, revealing their masses, accretion rates, and potential evolutionary stages, suggesting they are nearing the end of their active growth phase.

Contribution

It provides new measurements of black hole masses and accretion rates in z~2 radio galaxies, and compares their energy output with AGN feedback models, offering insights into galaxy and black hole co-evolution.

Findings

Black holes have masses of a few billion solar masses.

Black holes accrete at a few percent of the Eddington luminosity.

Host galaxy masses may have increased by up to a factor of 4 since z~2.

Abstract

We present an analysis of the AGN broad-line regions of 6 powerful radio galaxies at z>~2 (HzRGs) with rest-frame optical imaging spectroscopy obtained at the VLT. All galaxies have luminous (L(H-alpha)=few x 10^44 erg s^-1), spatially unresolved H-alpha line emission with FWHM>= 10,000 km s^-1 at the position of the nucleus, suggesting their AGN are powered by supermassive black holes with masses of few x 10^9 M_sun and accretion luminosities of a few percent of the Eddington luminosity. In two galaxies we also detect the BLRs in H-beta, suggesting relatively low extinction of A_V~1 mag, which agrees with constraints from X-ray observations. 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.