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 z0.1, 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_BH3x10^7M_o) in the nearby Universe, with a median Eddington ratio of ~0.2.

💡 Research Summary

This paper addresses the poorly constrained population of Compton‑thick (CT) active galactic nuclei (AGN) at low redshift (z ≈ 0.1) by combining X‑ray, mid‑infrared (mid‑IR), and optical diagnostics. The authors begin by cross‑matching the Sloan Digital Sky Survey (SDSS) Type‑2 AGN catalog with the XMM‑Newton Serendipitous Source Catalogue, selecting 147 objects that are undetected in the 2–10 keV band and satisfy f_X/f_