Details

Autor(en) / Beteiligte

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

Titel

Comoving Space Density of X-Ray-selected Active Galactic Nuclei

Ist Teil von

• The Astrophysical journal, 2005-05, Vol.624 (2), p.630-637

Ort / Verlag

Chicago, IL: IOP Publishing

Erscheinungsjahr

2005

Quelle

EZB Electronic Journals Library

Beschreibungen/Notizen

• 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 6 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 (61.8 deg super(2)) supplemented with 57 X-ray-bright AGNs from the Chandra Deep Field-North and Chandra Deep Field-South. Within our X-ray flux (f sub(0.3-8.0 keV) > 4 x 10 super(-15) ergs cm super(-2) s super(-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 L sub(X) > 44.5 ergs s super(-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 L sub(X) < 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.