High Redshift Quasars in the COSMOS Survey: The Space Density of z > 3 X-Ray Selected QSOs

We present a new measurement of the space density of high-redshift (z ≃ 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 F0.5–2keV > 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 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.