Comoving Space Density and Obscured Fraction of High-redshift Active Galactic Nuclei in the Subaru/XMM-Newton Deep Survey

Abstract

We study the comoving space density of X-ray-selected luminous active galactic nuclei (AGNs) and the obscured AGN fraction at high redshifts (3 < z < 5) in the Subaru/XMM-Newton Deep Survey field. From an X-ray source catalog with high completeness of optical identification thanks to deep optical images, we select a sample of 30 AGNs at z > 3 with intrinsic (de-absorbed and rest-frame 2-10 keV) luminosities of L _X = 10^44-45 erg s^-1 detected in the 0.5-2 keV band, consisting of 20 and 10 objects with spectroscopic and photometric redshifts, respectively. Utilizing the 1/V _max method, we confirm that the comoving space density of luminous AGNs decreases with redshift above z > 3. When combined with the Chandra-COSMOS result of Civano et al., the density decline of AGNs with L _X = 10^44-45 erg s^-1 is well represented by a power law of (1 + z)^{-6.2 ± 0.9}. We also determine the fraction of X-ray obscured AGNs with N _H > 10²² cm^-2 in the Compton-thin population to be 0.54^+0.17 _{- 0.19}, by carefully taking into account observational biases including the effects of photon statistics for each source. This result is consistent with an independent determination of the type-2 AGN fraction based on optical properties, for which the fraction is found to be 0.59 ± 0.09. Comparing our result with that obtained in the local universe, we conclude that the obscured fraction of luminous AGNs increases significantly from z = 0 to z > 3 by a factor of 2.5 ± 1.1.

Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.