Reliable Identification of Compton-thick Quasars at z ≈ 2: Spitzer Mid-Infrared Spectroscopy of HDF-oMD49

Many models that seek to explain the origin of the unresolved X-ray background predict that Compton-thick active galactic nuclei (AGNs) are ubiquitous at high redshift. However, few distant Compton-thick AGNs have been reliably identified to date. Here we present Spitzer IRS spectroscopy and 3.6-70 μm photometry of a z = 2.211 optically identified AGN (HDF-oMD49) that is formally undetected in the 2 Ms Chandra Deep Field-North (CDF-N) survey. The Spitzer IRS spectrum and spectral energy distribution of this object is AGN dominated, and a comparison of the energetics at X-ray wavelengths to those derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN is intrinsically luminous (L_{2-10 keV} ≈ 3 × 10⁴⁴ ergs s^-1) but heavily absorbed by Compton-thick material (N_Hgg 10²⁴ cm^-2) i.e., this object is a Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49, we found a further six objects at z ≈ 2-2.5 in the literature that are also X-ray weak/undetected but have evidence for AGN activity from optical and/or mid-IR spectroscopy, and show that all of these sources are likely to be Compton-thick quasars with L_{2-10 keV} > 10⁴⁴ ergs s^-1. On the basis of the definition of Daddi et al., these Compton-thick quasars would be classified as mid-IR excess galaxies, and our study provides the first spectroscopic confirmation of Compton-thick AGN activity in a subsample of these z ≈ 2 mid-IR-bright galaxies. Using the four objects that lie in the CDF-N field, we estimate the space density of reliably identified Compton-thick quasars [Φ ≈ (0.7-2.5) × 10^-5 Mpc^-3 for L_{2-10 keV} > 10⁴⁴ ergs s^-1 objects at z ≈ 2-2.5] and show that Compton-thick accretion was probably as ubiquitous as unobscured accretion in the distant universe.