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∼ 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 f_X/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, L_X≈ (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 N_H > 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 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≈ 3 × 10⁷ M_⊙) in the nearby Universe, with a median Eddington ratio of η≈ 0.2.