Comoving space density of X-ray-selected active galactic nuclei

J. D. Silverman, P. J. Green, W. A. Barkhouse, R. A. Cameron, C. Foltz, B. T. Jannuzi, D. W. Kim, M. Kim, A. Mossman, H. Tananbaum, B. J. Wilkes, M. G. Smith, R. C. Smith, P. S. Smith

Research output: Contribution to journalArticlepeer-review

58 Scopus citations


For measurement of the active galactic nucleus (AGN) luminosity function and its evolution, X-ray selection samples all types of AGNs and provides reduced obscuration bias in comparison with UV excess or optical surveys. The apparent decline in optically selected quasars above z ∼ 3 may be strongly affected by such a bias. The Chandra Multiwavelength Project (CHAMP) is characterizing serendipitously detected X-ray sources in a large number of fields with archival Chandra imaging. We present a preliminary measure of the comoving space density using a sample of 311 AGNs found in 23 CHAMP fields (∼1.8 deg2) supplemented with 57 X-ray-bright AGNs from the Chandra Deep Field-North and Chandra Deep Field-South. Within our X-ray flux (f0.3-8.0keV > 4 × 10-15 ergs cm-2 s-1) and optical magnitude (r′ < 22.5) limits, our sample includes 14 broad emission-line AGNs at z > 3. Using this X-ray-selected sample, we detect a turnover in the comoving space density of luminous type 1 AGNs (log LX > 44.5 ergs s-1, measured in the 0.3-8.0 keV band and corrected for Galactic absorption) at z > 2.5. Our X-ray sample is the first to show a behavior similar to the well-established evolution of the optical quasar luminosity function. A larger sample of high-redshift AGNs and with a greater fraction of identified sources, either spectroscopic or photometric, at faint optical magnitudes (r′ > 22.5) are required to remove the remaining uncertainty in our measure of the X-ray luminosity function, particularly given the possibility that AGNs might be more easily obscured optically at high redshift. We confirm that for z < 1, lower luminosity AGNs (log LX < 44.5) are more prevalent by more than an order of magnitude than those with high luminosity. We have combined the Chandra sample with AGNs from the ROSAT surveys to present a measure of the space density of luminous type 1 AGNs in the soft X-ray band (0.5-2.0 keV) that confirms the broadband turnover described above.

Original languageEnglish (US)
Pages (from-to)630-637
Number of pages8
JournalAstrophysical Journal
Issue number2 I
StatePublished - May 10 2005


  • Galaxies: active
  • Quasars: general
  • Surveys
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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