Comoving Space Density of X‐Ray–selected Active Galactic Nuclei

For measurement of the AGN luminosity function and its evolution, X-rayselection samples all types of AGN and provides reduced obscuration bias incomparison with UV-excess or optical surveys. The apparent decline inoptically-selected quasars above z~3 may be strongly affected by such a bias.The Chandra Multiwavelength Project (ChaMP) is characterizing serendipitouslydetected X-ray sources in a large number of fields with archival Chandraimaging. We present a preliminary measure of the co-moving space density usinga sample of 311 AGN found in 23 ChaMP fields (~1.8 deg^2) supplemented with 57X-ray bright AGN from the CDF-N and CDF-S. Within our X-ray flux (f(0.3-8.0keV) > 4x10^-15 erg cm^-2 s^-1) and optical magnitude (r' < 22.5) limits, oursample includes 14 broad emission line AGN at z > 3. Using this X-ray selectedsample, we detect a turnover in the co-moving space density of luminous type 1AGN (log Lx > 44.5; units erg s^-1; measured in the 0.3-8.0 keV band andcorrected for Galactic absorption) at z > 2.5. Our X-ray sample is the first toshow a behavior similar to the well established evolution of the optical quasarluminosity function. A larger sample of high redshift AGN and with a greaterfraction of identified sources, either spectroscopic or photometric, at faintoptical magnitudes (r' > 22.5) are required to remove the remaining uncertaintyin our measure of the X-ray luminosity function. We confirm that for z < 1,lower luminosity AGN (log Lx < 44.5) are more prevalent by more than an orderof magnitude than those with high luminosity. We have combined the Chandrasample with AGN from the ROSAT surveys to present a measure of the spacedensity of luminous type 1 AGN in the soft X-ray band (0.5-2.0 keV) whichconfirms the broad band turnover described above.Comment: 8 pages, 9 figures, 1 color figure, Expanded version accepted by Ap