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Searching for Compton-thick active galactic nuclei at z similar to 0.1

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journal contribution
posted on 25.09.2012, 13:59 by A.D. Goulding, D.M. Alexander, J.R. Mullaney, J.M. Gelbord, R.C. Hickox, M. Ward, Michael G. Watson
Using a suite of X-ray, mid-infrared (mid-IR) and optical active galactic nuclei (AGN) luminosity indicators, we search for Compton-thick AGNs with intrinsic L-X > 1042 erg s-1 at z similar to 0.03-0.2, a region of parameter space which is currently poorly constrained by deep narrow-field and high-energy (E > 10 keV) all-sky X-ray surveys. We have used the widest XMM-Newton survey (the serendipitous source catalogue) to select a representative subsample (14; approximate to 10 per cent) of the 147 X-ray undetected candidate Compton-thick AGNs in the Sloan Digital Sky Survey (SDSS) with f(X)/f([O III]) < 1; the 147 sources account for approximate to 50 per cent of the overall type-2 AGN population in the SDSS-XMM overlap region. We use mid-IR spectral decomposition analyses and emission-line diagnostics, determined from pointed Spitzer-InfraRed Spectrograph spectroscopic observations of these candidate Compton-thick AGNs, to estimate the intrinsic AGN emission [predicted 2-10 keV X-ray luminosities, L-X approximate to (0.2-30) x 10(42) erg s(-1)]. On the basis of the optical [O III], mid-IR [O IV] and 6 - mu m AGN continuum luminosities, we conservatively find that the X-ray emission in at least 6/14 (greater than or similar to 43 per cent) of our sample appears to be obscured by Compton-thick material with N-H > 1.5 x 10(24) cm(-2). Under the reasonable assumption that our 14 AGNs are representative of the overall X-ray undetected AGN population in the SDSS-XMM parent sample, we find that greater than or similar to 20 per cent of the optical type-2 AGN population are likely to be obscured by Compton-thick material. This implies a space density of log(Phi) greater than or similar to -4.9Mpc(-3) for Compton-thick AGNs with L-X greater than or similar to 10(42) erg s(-1) at z similar to 0.1, which we suggest may be consistent with that predicted by X-ray background synthesis models. Furthermore, using the 6-mu m continuum luminosity to infer the intrinsic AGN luminosity and the stellar velocity dispersion to estimate M-BH, we find that the most conservatively identified Compton-thick AGNs in this sample may harbour some of the most rapidly growing black holes (median M-BH approximate to 3 x 10(7) M-circle dot) in the nearby Universe, with a median Eddington ratio of eta approximate to 0.2.

History

Citation

Monthly Notices of the Royal Astronomical Society, 2011, 411 (2), pp. 1231-1244 (14)

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society

Publisher

Blackwell Publishing on behalf of the Royal Astronomical Society

issn

0035-8711

Copyright date

2010

Available date

25/09/2012

Publisher version

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2010.17755.x/abstract

Language

en