Comoving space density and obscured fraction of high-redshift active galactic nuclei in the Subaru/XMM-Newton deep survey

Kazuo Hiroi, Yoshihiro Ueda, Masayuki Akiyama, Mike G. Watson

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24 Citations (Scopus)


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 2-10keV) luminosities of LX = 10 44-45ergs-1 detected in the 0.5-2keV band, consisting of 20 and 10 objects with spectroscopic and photometric redshifts, respectively. Utilizing the 1/Vmax 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 etal., the density decline of AGNs with LX = 1044-45ergs-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 NH > 1022cm-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.

Original languageEnglish
Article number49
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 2012 Oct 10


  • X-rays: galaxies
  • galaxies: active
  • galaxies: evolution
  • surveys

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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