Searching for Compton-thick active galactic nuclei at z∼ 0.1

Abstract

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 > 1042ergs-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 > 10keV) 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[Oiii] < 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 2-10keV X-ray luminosities, LX≈ (0.2-30) × 1042 ergs-1]. On the basis of the optical [Oiii], mid-IR [Oiv] 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. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.