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Title:
Reliable Identification of Compton-thick Quasars at z ~ 2: Spitzer Mid-Infrared Spectroscopy of HDF-oMD49
Authors:
Alexander, D. M.; Chary, R.-R.; Pope, A.; Bauer, F. E.; Brandt, W. N.; Daddi, E.; Dickinson, M.; Elbaz, D.; Reddy, N. A.
Affiliation:
AA(Department of Physics, Durham University, Durham DH1 3LE, UK.), AB(Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125.), AC(Spitzer Fellow.; Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada.; National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719.), AD(Chandra Fellow; Columbia Astrophysics Laboratory, Columbia University, Pupin Laboratories, 550 West 120th Street, New York, NY 10027.), AE(Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802.), AF(Laboratoire AIM, CEA/DSM-CNRS-Universitè Paris Diderot, Irfu, Orme des Merisiers, 91191 Gif-sur-Yvette, France.), AG(National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719.), AH(Laboratoire AIM, CEA/DSM-CNRS-Universitè Paris Diderot, Irfu, Orme des Merisiers, 91191 Gif-sur-Yvette, France.), AI(National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719.)
Publication:
The Astrophysical Journal, Volume 687, Issue 2, pp. 835-847. (ApJ Homepage)
Publication Date:
11/2008
Origin:
UCP
ApJ Keywords:
Galaxies: Active, Galaxies: High-Redshift, Infrared: Galaxies, Ultraviolet: Galaxies, X-Rays: Galaxies
DOI:
10.1086/591928
Bibliographic Code:
2008ApJ...687..835A

Abstract

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 (L2-10keV~3×1044 ergs s-1) but heavily absorbed by Compton-thick material (NH>>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 L2-10keV>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 L2-10keV>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.
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