High-Redshift Quasars in the COSMOS Survey: The Space Density of z > 3 X-Ray Selected QSOs
Ilbert, O.; Salvato, M.; Merloni, A.; Fiore, F.; Vignali, C.; Gilli, R.; Comastri, A.; Iwasawa, K.; Cappelluti, N.; Brusa, M.; Silverman, J.; Hasinger, G.; Mainieri, V.; Capak, P.; Civano, F.; Elvis, M.; Lilly, S.; Trump, J.; Mignoli, M.; Zamorani, G.; Impey, C.; Bongiorno, A.
Germany, Italy, United States, Switzerland
Abstract
We present a new measurement of the space density of high-redshift (zsime 3.0-4.5), X-ray-selected quasi-stellar objects (QSOs) obtained by exploiting the deep and uniform multiwavelength coverage of the COSMOS survey. We have assembled a large (40 objects), homogeneous sample of z > 3 QSOs with X-ray flux F 0.5-2 keV > 10-15 erg cm-2 s-1, and available spectroscopic (22) or photometric (18) redshifts. We discuss their optical (color-color diagrams) and X-ray properties, their number counts and space densities and compare our findings with previous works and model predictions. We find that the optical properties of X-ray-selected quasars are not significantly different from those of optically selected samples. There is evidence for substantial X-ray absorption (logNH > 23 cm-2) in about 20% of the sources in the sample. We find that the comoving space density of luminous (LX gsim 1044 erg s-1) QSOs declines exponentially (by an e-folding per unit redshift) in the z~ 3.0-4.5 range, with a behavior similar to that observed for optically bright unobscured QSOs selected in large area optical surveys. Prospects for future, large and deep X-ray surveys are also discussed.