The XMM-Newton wide-field survey in the COSMOS field. III. Optical identification and multiwavelength properties of a large sample of X-ray-selected sources

We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg ² of the COSMOS XMM-Newton survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ∼10 ^-15 erg cm ^-2 s ^-1 (∼5 × 10 ^-15 erg cm ^-2 s ^-1). In order to identify the correct optical counterparts and to assess the statistical significance of the X-ray - to - optical associations we have used the "likelihood ratio technique." Here we present the identification method and its application to the CFHT I-band and photometric catalogs. We were able to associate a candidate optical counterpart to ∼90% (626) of the X-ray sources, while for the remaining ∼10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (I _AB > 25) or to the presence of multiple optical sources, with similar likelihoods of being the correct identification, within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS, and literature spectroscopic data; the spectroscopic sample comprises 248 objects (∼40% of the full sample). Our analysis of this statistically meaningful sample of X-ray sources reveals that for ∼80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical - to - near-infrared colors: the large majority of spectroscopically identified broad-line active galactic nuclei (BL AGNs) have a pointlike morphology on ACS data, blue optical colors in color-color diagrams, and an X-ray - to - optical flux ratio typical of optically selected quasars. Conversely, sources classified as narrow line AGNs or normal galaxies are on average associated with extended optical sources, have significantly redder optical - to - near-infrared colors, and span a larger range of X-ray - to - optical flux ratios. However, about 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the "extended" BL AGNs lie at redshift < 1.5, while the redshift distribution of the full BL AGN population peaks at z ∼ 1.5. The most likely explanation is that in these objects the nuclear emission is not dominant with respect to the host galaxy emission in the observed ACS band. Our analysis also suggests that the type 2/type 1 ratio decreases toward high luminosities, in qualitative agreement with the results from X-ray spectral analysis and the most recent modeling of the X-ray luminosity function evolution.