Civano, F.F.CivanoBrusa, M.M.BrusaComastri, A.A.ComastriPuccetti, SimonettaSimonettaPuccetti2020-09-172020-09-172011-11-01https://hdl.handle.net/20.500.13025/2245The Astrophysical Journal, Volume 741, Issue 2, article id. 91, <NUMPAGES>11</NUMPAGES> pp. (2011).We present the high-redshift (3 <z < 5.3) 0.5-2 keV number counts and the 2-10 keV (rest-frame) space density of X-ray-selected active galactic nuclei (AGNs) detected in the Chandra Cosmic Evolution Survey. The sample comprises 81 X-ray-detected sources with available spectroscopic (31) and photometric (50) redshifts plus 20 sources with a formal z <SUB>phot</SUB> < 3 but with a broad photometric redshift probability distribution, such that z <SUB>phot</SUB> + 1sigma > 3. Eighty-one sources are selected in the 0.5-2 keV band, fourteen are selected in the 2-10 keV and six in the 0.5-10 keV bands. We sample the high-luminosity (log L <SUB>(2-10 keV)</SUB> > 44.15 erg s<SUP>-1</SUP>) space density up to z ~ 5 and a fainter luminosity range (43.5 erg s<SUP>-1</SUP> < log L <SUB>(2-10 keV)</SUB> < 44.15 erg s<SUP>-1</SUP>) than previous studies, up to z = 3.5. We weighted the contribution to the number counts and the space density of the sources with photometric redshift by using their probability of being at z > 3. We find that the space density of high-luminosity AGNs declines exponentially at all the redshifts, confirming the trend observed for optically selected quasars. At lower luminosity, the measured space density is not conclusive, and a larger sample of faint sources is needed. Comparisons with optical luminosity functions and black hole formation models are presented together with prospects for future surveys.evolutiongalaxies activesurveysX-rays galaxies10.1088/0004-637X/741/2/91http://adsabs.harvard.edu/abs/2011ApJ...741...91C2011ApJ...741...91C