Abstract

We report the final optical identifications of the medium-depth (similar to 60 ks), contiguous (2 deg(2)) XMM-Newton survey of the COSMOS field. XMM-Newton has detected similar to 1800 X-ray sources down to limiting fluxes of similar to 5 x 10(-16), similar to 3 x 10(-15), and similar to 7 x 10(-15) erg cm(-2) s(-1) in the 0.5-2 keV, 2-10 keV, and 5-10 keV bands, respectively (similar to 1 x 10(-15), similar to 6 x 10(-15), and similar to 1 x 10(-14) erg cm(-2) s(-1), in the three bands, respectively, over 50% of the area). The work is complemented by an extensive collection of multiwavelength data from 24 mu m to UV, available from the COSMOS survey, for each of the X-ray sources, including spectroscopic redshifts for greater than or similar to 50% of the sample, and high-quality photometric redshifts for the rest. The XMM and multiwavelength flux limits are well matched: 1760 (98%) of the X-ray sources have optical counterparts, 1711 (similar to 95%) have IRAC counterparts, and 1394 (similar to 78%) have MIPS 24 mu m detections. Thanks to the redshift completeness (almost 100%) we were able to constrain the high-luminosity tail of the X-ray luminosity function confirming that the peak of the number density of log L(X) > 44.5 active galactic nuclei (AGNs) is at z similar to 2. Spectroscopically identified obscured and unobscured AGNs, as well as normal and star-forming galaxies, present well-defined optical and infrared properties. We devised a robust method to identify a sample of similar to 150 high-redshift (z > 1), obscured AGN candidates for which optical spectroscopy is not available. We were able to determine that the fraction of the obscured AGN population at the highest (L(X) > 10(44) erg s(-1)) X-ray luminosity is similar to 15%-30% when selection effects are taken into account, providing an important observational constraint for X-ray background synthesis. We studied in detail the optical spectrum and the overall spectral energy distribution of a prototypical Type 2 QSO, caught in a stage transitioning from being starburst dominated to AGN dominated, which was possible to isolate only thanks to the combination of X-ray and infrared observations.