Searching for Compton-thick active galactic nuclei at z~0.1
  Using a suite of X-ray, mid-IR and optical active galactic nuclei (AGN)
luminosity indicators, we search for Compton-thick (CT) AGNs with intrinsic
L_X>10^42erg/s 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 sub-sample (14; ~10%) of the 147 X-ray
undetected candidate CT AGNs in the Sloan Digital Sky Survey (SDSS) with
f_X/f_[OIII]<1; the 147 sources account for ~50% 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-IRS
spectroscopic observations of these candidate CT AGNs, to estimate the
intrinsic AGN emission (predicted L_X,2-10keV (0.2-30)x10^42erg/s). On the
basis of the optical [OIII], mid-IR [OIV] and 6um AGN continuum luminosities we
conservatively find that the X-ray emission in at least 6/14 (>43%) of our
sample appear to be obscured by CT material with N_H>1.5x10^24cm^-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 >20% of
the optical Type-2 AGN population are likely to be obscured by CT material.
This implies a space-density of log(Phi) >-4.9Mpc^-3 for CT AGNs with
L_X>10^42erg/s at z~0.1, which we suggest may be consistent with that predicted
by X-ray background synthesis models. Furthermore, using the 6um continuum
luminosity to infer the intrinsic AGN luminosity and the stellar velocity
dispersion to estimate M_BH, we find that the most conservatively identified CT
AGNs in this sample may harbour some of the most rapidly growing black holes
(median M_BH~3x10^7M_o) in the nearby Universe, with a median Eddington ratio
of ~0.2.
