Low accretion rates at the AGN cosmic downsizing epoch

¹ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK e-mail: ababic@astro.ox.ac.uk
² Centre for Astrophysics Research, STRI, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK
³ Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ X-Ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Greenbelt, MD 20771, USA
⁵ Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁶ School of Physics A28, University of Sydney, NSW 2006, Australia

Received: 16 July 2007
Accepted: 5 September 2007

Abstract

Context. X-ray surveys of Active Galactic Nuclei (AGN) indicate “cosmic downsizing”, with the comoving number density of high-luminosity objects peaking at higher redshifts (z~2) than low-luminosity AGN (z < 1).

Aims. We test whether downsizing is caused by activity shifting towards low-mass black holes accreting at near-Eddington rates, or by a change in the average rate of accretion onto supermassive black holes. We estimate the black hole masses and Eddington ratios of an X-ray selected sample of AGN in the Chandra Deep Field South at z < 1, probing the epoch where AGN cosmic downsizing has been reported.

Methods. Black hole masses are estimated both from host galaxy stellar masses, which are estimated from fitting to published optical and near-infrared photometry, and from near-infrared luminosities, applying established correlations between black hole mass and host galaxy properties. Both methods give consistent results. Comparison and calibration of possible redshift-dependent effects is also made using published faint host galaxy velocity dispersion measurements.

Results. The Eddington ratios in our sample span the range ~10^-5-1, with median = -2.87, and with typical black hole masses ~ 10⁸ . The broad distribution of Eddington ratios is consistent with that expected for AGN samples at low and moderate luminosity. We find no evidence that the CDF-S AGN population is dominated by low-mass black holes accreting at near-Eddington ratios and the results suggest that diminishing accretion rates onto average-sized black holes are responsible for the reported AGN downsizing at redshifts below unity.