Publication: Infrared Excess sources: Compton thick QSOs, low-luminosity Seyferts or starbursts?
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Publication Date
2010-07-21
Authors
Georgakakis, A.
Rowan-Robinson, M.
Nandra, K.
Digby-North, J.
Barro, G.
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Wiley
Abstract
We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous [L_X(2–10 keV) > 10^43 erg s^−1] Compton thick (NH > 2 × 1024 cm−2) QSOs at z≈ 2. Lower redshift, z≈ 1, analogues of the distant IRX population are identified by first redshifting to z= 2 the spectral energy distributions (SEDs) of all sources with secure spectroscopic redshifts in the AEGIS (6488) and the GOODS-North (1784) surveys and then selecting those that qualify as IRX sources at that redshift. A total of 19 galaxies are selected. The mean redshift of the sample is z≈ 1. We do not find strong evidence for Compton thick QSOs in the sample. For nine sources with X-ray counterparts, the X-ray spectra are consistent with Compton thin active galactic nucleus (AGN). Only three of them show tentative evidence for Compton thick obscuration. The SEDs of the X-ray undetected population are consistent with starburst activity. There is no evidence for a hot dust component at the mid-infrared associated with AGN heated dust. If the X-ray undetected sources host AGN, an upper limit of L_X(2–10 keV) = 10^43 erg s^−1 is estimated for their intrinsic luminosity. We propose that a large fraction of the z≈ 2 IRX population is not Compton thick quasi-stellar objects (QSOs) but low-luminosity [L_X(2–10 keV) < 10^43 erg s^−1], possibly Compton thin, AGN or dusty starbursts. It is shown that the decomposition of the AGN and starburst contribution to the mid-IR is essential for interpreting the nature of this population, as star formation may dominate this wavelength regime.
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© 2010 The Authors.
Journal compilation, © 2010 RAS.
The authors wish to thank the anonymous referee for constructive suggestions that improved the paper and Alison Coil for useful comments. AG acknowledges financial support from the Marie-Curie Reintegration Grant PERG03-GA-2008-230644. PGP-G acknowledges support from the Spanish Ministry of Science and Innovation under grants AYA 2006-02358, AYA 2006-15698-C02-02 and CSD2006-100070, and from the Ramón y Cajal Program financed by the Spanish Government and the European Union. This study makes use of data from AEGIS, a multiwavelength sky survey conducted with the Chandra, GALEX, Hubble, Keck, CFHT, MMT, Subaru, Palomar, Spitzer, VLA and other telescopes and supported in part by the NSF, NASA and the STFC.
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