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

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.