2008/03/31 by D. M. Alexander, Ranga‐Ram Chary, R. R. Chary +10 · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena #astro-ph
paper · pdf · doi:10.1086/591928
ApJ in press; 13 pages, 7 figures, 1 table. Paper updated to take account of feedback, predominantly exploring other X-ray-mid-IR relationships, providing an N_H constraint from the X-ray data, and adding a second panel to Fig 1 to compare to the composite mid-IR spectrum of X-ray obscured Compton-thin quasars. Results and conclusions are unchanged
arxiv created 2008/07/15 · openalex publication_date 2008/10/31 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/04
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 × 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 ≈ 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 > 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 ≈ 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 > 10 44 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.