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Reliable identification of Compton-thick quasars at z~2 : Spitzer mid-infrared spectroscopy of HDF-oMD49.

Alexander, D.M. and Chary, R.R. and Pope, A. and Bauer, F.E. and Brandt, W.N. and Daddi, E. and Dickinson, M. and Elbaz, D. and Reddy, N.A. (2008) 'Reliable identification of Compton-thick quasars at z~2 : Spitzer mid-infrared spectroscopy of HDF-oMD49.', Astrophysical journal., 687 (2). pp. 835-847.


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.

Item Type:Article
Keywords:Galaxies, Active galaxies, High-redshift, Infrared, Ultraviolet, X-rays.
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Publisher statement:© 2008. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
Date accepted:No date available
Date deposited:04 September 2013
Date of first online publication:2008
Date first made open access:No date available

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