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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
Low accretion rates at the AGN cosmic downsizing epoch
Astronomy and Astrophysics, Volume 474, No. 3, Year 2007
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Description
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 log(Lbol/LEdd) = -2.87, and with typical black hole masses MBH ∼ 108 M⊙. 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. © ESO 2007.
Authors & Co-Authors
Babić, Ana
United Kingdom, Oxford
University of Oxford
Miller, Lance R.
United Kingdom, Oxford
University of Oxford
Jarvis, Matthew J.
United Kingdom, Hatfield
University of Hertfordshire
Turner, Tracey Jane
United States, Baltimore
University of Maryland, Baltimore County Umbc
United States, Greenbelt
Nasa Goddard Space Flight Center
Alexander, David M.
United Kingdom, Durham
Durham University
Croom, Scott M.
Australia, Sydney
The University of Sydney
Statistics
Citations: 59
Authors: 6
Affiliations: 6
Identifiers
Doi:
10.1051/0004-6361:20078286
ISSN:
14320746
Study Design
Cross Sectional Study