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, BelindaSmith, M. G.Smith, R. C.Smith, P. S.2017-02-132005Silverman, J. D., P. J. Green, W. A. Barkhouse, R. A. Cameron, C. Foltz, B. T. Jannuzi, D.‐W. Kim, et al. 2005. “Comoving Space Density of X‐Ray–selected Active Galactic Nuclei.” The Astrophysical Journal 624 (2) (May 10): 630–637. doi:10.1086/429361.0004-637Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:30212135For 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 (f0.3-8.0 keV > 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.en-USGalaxies: activeQuasars: generalSurveysX-rays: galaxiesJournal Article2017-02-1310.1086/429361