THE XMM-NEWTON WIDE-FIELD SURVEY IN THE COSMOS FIELD (XMM-COSMOS): DEMOGRAPHY AND MULTIWAVELENGTH PROPERTIES OF OBSCURED AND UNOBSCURED LUMINOUS ACTIVE GALACTIC NUCLEI*

摘要

We report the final optical identifications of the medium-depth (~60?ks), contiguous (2?deg2) XMM-Newton survey of the COSMOS field. XMM-Newton has detected ~1800 X-ray sources down to limiting fluxes of ~5 × 10–16, ~3 × 10–15, and ~7 × 10–15?erg?cm–2?s–1 in the 0.5-2?keV, 2-10?keV, and 5-10?keV bands, respectively (~1 × 10–15, ~6 × 10–15, and ~1 × 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 μm to UV, available from the COSMOS survey, for each of the X-ray sources, including spectroscopic redshifts for 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 (~95%) have IRAC counterparts, and 1394 (~78%) have MIPS 24?μm 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 LX 44.5 active galactic nuclei (AGNs) is at z ~ 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 ~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 (LX 1044?erg?s–1) X-ray luminosity is ~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.