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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. ISSN 0004-637X. doi:10.1086/591928.

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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 × 10^44 ergs s^−1) but heavily absorbed by Compton-thick material (NH » 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 L2–10 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 approx 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 approx (0.7–2.5) × 10^−5 Mpc^−3 for L2–10 keV > 10^44 ergs s^−1 objects at z approx 2–2.5] and show that Compton-thick accretion was probably as ubiquitous as unobscured accretion in the distant universe.

Item Type:Article
Related URLs:
URLURL TypeDescription
Alexander, D. M.0000-0002-5896-6313
Chary, R.-R.0000-0001-7583-0621
Pope, A.0000-0001-8592-2706
Bauer, F. E.0000-0002-8686-8737
Brandt, W. N.0000-0002-0167-2453
Daddi, E.0000-0002-3331-9590
Dickinson, M.0000-0001-5414-5131
Elbaz, D.0000-0002-7631-647X
Reddy, N. A.0000-0001-9687-4973
Additional Information:© 2008. The American Astronomical Society. Received 2008 March 10; accepted 2008 July 10; published 2008 November 10. Print publication: Issue 2 (2008 November 10). We acknowledge support from the Royal Society (D.M.A.), the Spitzer Space Telescope Fellowship program (A.P.), the Natural Sciences and Engineering Research Council of Canada and the Canadian Space Agency (A.P.), the Chandra Fellowship program (F.E.B.), and the NASA LTSA grant NAG5-13035 (W.N.B.). We thank R. Gilli for generously providing his model tracks and Compton-thick AGN spectra, and for his insightful comments on an earlier draft of this paper. We also thank F. Fiore for providing the data from his sample; E. Sturm for providing the composite obscured quasar spectrum; C. Done, M. Polletta, and E. Treister for scientific feedback; and the referee for useful suggestions. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and Ames Research Center.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Canadian Space Agency (CSA)UNSPECIFIED
Subject Keywords:galaxies: active; galaxies: high-redshift; infrared: galaxies; ultraviolet: galaxies; X-rays: galaxies
Issue or Number:2
Record Number:CaltechAUTHORS:ALEapj08
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13037
Deposited By: Archive Administrator
Deposited On:16 Jan 2009 05:49
Last Modified:08 Nov 2021 22:35

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