2006ApJ...652L..79T

In order to study the evolution of the relative fraction of obscured active galactic nuclei (AGNs), we constructed the largest sample to date of AGNs selected in hard X-rays. The full sample contains 2341 X-ray-selected AGNs, roughly 4 times the largest previous samples studied in this connection. Of these, 1229 (53%) have optical counterparts for which redshifts are available; these span the redshift range z=0-4. The observed fraction of obscured AGNs declines only slightly with redshift. Correcting for selection bias, we find that the intrinsic fraction of obscured AGNs must actually increase with redshift, as (1+z)^α, with α≃0.4±0.1. This evolution is consistent with the integrated X-ray background, which provides the strongest constraints at relatively low redshift, z∼1. Summing over all AGNs, we estimate the bolometric AGN light to be 3.8 nW/m2/sr, or ≲8% of the total extragalactic light. Together with the observed black hole mass density in the local universe, this implies an accretion efficiency of η∼0.1-0.2, consistent with the values typically assumed.