AGN population in the deepest hard X-ray extragalactic survey

Paltani, S., Courvoisier, T.J.L., Dwelly, T., McHardy, I.M. and Walter, R. (2008) AGN population in the deepest hard X-ray extragalactic survey. arXiv, 12-[pp].

Abstract

We present the results of the analysis of the AGN population in the deepest extragalactic hard X-ray survey. The survey is based on INTEGRAL observation of the 3C 273/Coma cluster region, and covers 2500 deg² with a 20-60 keV flux limit 1.5 times lower than other surveys at similar energies, resolving about 2.5% of the cosmic hard X-ray background. Using this survey, we can constrain in an unbiased way the distribution of hydrogen column absorption up to N_h=10²⁵ cm^-2. We put an upper limit of 24% to the fraction of Compton-thick objects. Compared to models of the AGN population selected in the 2-10 keV band, the Log N-Log S diagram is generally in good agreement, but the N_h distribution is significantly different, with significantly less unabsorbed sources (N_h < 10²² cm^-2) at a given flux limit compared to the models. We also study the local hard X-ray luminosity function (LF), which is compatible with what is found in other recent hard X-ray surveys. The extrapolation of the 2-10 keV LF is lower than the hard X-ray LF. The discrepancy is resolved if AGN spectra typically present reflection humps with reflection fraction R ~ 1. Finally, we use the population properties of this survey to show that a future ultra-deep INTEGRAL extragalactic survey can result in a quite large AGN sample with enough objects at redshifts larger than z = 0.05 so that we can detect evolution in the hard X-ray LF.

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