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Infrared Excess sources: Compton thick QSOs, low-luminosity Seyferts or starbursts?



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Georgakakis, A. and Rowan-Robinson, M. and Nandra, K. and Digby-North, J. and Pérez González, Pablo Guillermo and Barro, G. (2010) Infrared Excess sources: Compton thick QSOs, low-luminosity Seyferts or starbursts? Monthly notices of the Royal Astronomical Society, 406 (1). pp. 420-433. ISSN 0035-8711

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Official URL: http://dx.doi.org/10.1111/j.1365-2966.2010.16727.x


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.

Item Type:Article
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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.

Uncontrolled Keywords:Active galactic nuclei; Dust-obscured galaxies; Deep-field-North; Extended groth strip; Similar-to 2; Spitzer midinfrared spectroscopy; Spectral energy-distributions; Star-forming galaxies; XMM-Newton; Photometric redshifts
Subjects:Sciences > Physics > Astrophysics
Sciences > Physics > Astronomy
ID Code:38541
Deposited On:21 Jul 2016 15:44
Last Modified:10 Dec 2018 15:05

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