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 mu m 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 (L2-10keV approximate to 3 x 10(44) ergs s(-1)) but heavily absorbed by Compton-thick material (N-H >> 10(24) 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 approximate to 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 L2-10keV > 10(44) 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 approximate to 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 [phi approximate to (0.7-2.5) x 10(-5) Mpc(-3) for L2-10keV > 10(44) ergs s(-1) objects at z approximate to 2-2.5] and show that Compton- thick accretion was probably as ubiquitous as unobscured accretion in the distant universe.