X-ray detected infrared excess AGN in the Chandra deep fields: a moderate fraction of Compton-thick sources^⋆

¹ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Italy
e-mail: ig@astro.noa.gr
² Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
³ Max Planck Institut für Extraterestrische Physik, Garching bei München, Germany

Received: 12 March 2010
Accepted: 13 July 2010

Abstract

We examine the properties of the X-ray-detected infrared excess AGN or dust obscured galaxies (DOGs) in the Chandra deep fields (CDFs). We find 26 X-ray-detected sources that obey the 24 μm to R-band flux ratio criterion f₂₄/f_R > 1000. These are at a median redshift of 2.3, while their IR luminosities are above 10¹²   L_⊙. Their X-ray luminosities are all above a few times 10⁴² erg   s^-1 in the 2–10 keV band, which unambiguously argues that they host AGN. Nevertheless, their IR spectral energy distributions are split between AGN (Mrk231) and star-forming templates (Arp220). Our primary goal is to examine their individual X-ray spectra in order to assess whether this X-ray-detected DOG population contains heavily obscured or even Compton-thick sources. The X-ray spectroscopy reveals a mixed bag of objects. We find that four out of the 12 sources with adequate photon statistics and hence reliable X-ray spectra show evidence for a hard X-ray spectral index with Γ ~ 1 or harder, which is consistent with a Compton-thick spectrum. In total 12 out of the 26 DOGs show evidence for flat spectral indices. However, owing to the limited photon statistics we cannot distinguish whether they are flat because they are reflection-dominated or because they show moderate amounts of absorption. Seven DOGs show relatively steep spectra Γ > 1.4, indicative of low column densities. All the above suggests a fraction of Compton-thick sources that does not exceed 50%. The average X-ray spectrum of all 26 DOGs is hard (Γ ~ 1.1) or even harder (Γ ~ 0.6) when we exclude the brightest sources. These spectral indices agree with the stacked spectrum of X-ray-undetected sources (Γ ≈ 0.8 in the CDFN). This could suggest (but not necessarily prove) that X-ray undetected DOGs, in a similar way as the X-ray-detected ones presented here, are hosting a moderate fraction of Compton-thick sources.