Journal Article

Searching for Compton-thick active galactic nuclei at <i>z</i>∼ 0.1

A. D. Goulding, D. M. Alexander, J. R. Mullaney, J. M. Gelbord, R. C. Hickox, M. Ward and M. G. Watson

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 411, issue 2, pages 1231-1244
Published in print February 2011 | ISSN: 0035-8711
Published online February 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17755.x
Searching for Compton-thick active galactic nuclei at z∼ 0.1

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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 LX > 1042 erg s−1 at z∼ 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; ≈10 per cent) of the 147 X-ray undetected candidate Compton-thick AGNs in the Sloan Digital Sky Survey (SDSS) with fX/f[O iii] < 1; the 147 sources account for ≈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, LX≈ (0.2–30) × 1042 erg s−1]. On the basis of the optical [O iii], mid-IR [O iv] and 6 −μ m AGN continuum luminosities, we conservatively find that the X-ray emission in at least 6/14 ( per cent) of our sample appears to be obscured by Compton-thick material with NH > 1.5 × 1024 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 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 for Compton-thick AGNs with at z∼ 0.1, which we suggest may be consistent with that predicted by X-ray background synthesis models. Furthermore, using the 6 −μ m continuum luminosity to infer the intrinsic AGN luminosity and the stellar velocity dispersion to estimate MBH, 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≈ 3 × 107 M) in the nearby Universe, with a median Eddington ratio of η≈ 0.2.

Keywords: galaxies: active; galaxies: evolution; galaxies: nuclei; infrared: galaxies; X-rays: galaxies

Journal Article.  10833 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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