Reliable identification of compton-thick quasars at z ≈2: Spitzer mid-infrared spectroscopy of HDF-oMD49

D. M. Alexander, R. R. Chary, A. Pope, F. E. Bauer, W. N. Brandt, E. Daddi, M. Dickinson, D. Elbaz, N. A. Reddy

Research output: Contribution to journalArticlepeer-review

113 Scopus citations


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 (L2-10 keV ≈ 3 × 1044 ergs s-1) but heavily absorbed by Compton-thick material (N H ≫ 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 L2-10 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 [Ψ ≈ (0.7-2.5) × 10-5 Mpc-3 for L2-10 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.

Original languageEnglish (US)
Pages (from-to)835-847
Number of pages13
JournalAstrophysical Journal
Issue number2
StatePublished - Nov 10 2008

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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