Optical identification of XMM sources in the CFHTLS

TL;DR

This study provides extensive optical spectroscopic identifications of X-ray sources from the XMM-LSS survey within the CFHTLS, revealing the nature, redshift distribution, and spectral properties of these sources, significantly increasing the known identifications in this field.

Contribution

It offers the first large-scale spectroscopic identification of XMM-LSS sources in the CFHTLS, expanding the catalog and analyzing the properties of AGNs, galaxies, and stars in X-ray surveys.

Findings

Approximately 59% of sources were reliably identified spectroscopically.

About 55% are broad-line AGNs with redshifts up to 3.87.

Optical-to-X-ray spectral index correlates with luminosities, not redshift.

Abstract

We present optical spectroscopic identifications of X-ray sources in ~3 square degrees of the XMM-Large Scale Structure survey (XMM-LSS), also covered by the Canada France Hawaii Telescope Legacy Survey (CFHTLS), obtained with the AAOmega instrument at the Anglo Australian Telescope. In a flux limited sample of 829 point like sources in the optical band with g' <~22 mag and the 0.5-2 keV flux > 1x10^{-15}erg/cm^2/s, we observed 695 objects and obtained reliable spectroscopic identification for 489 sources, ~59% of the overall sample. We therefore increase the number of identifications in this field by a factor close to five. Galactic stellar sources represent about 15% of the total (74/489). About 55% (267/489) are broad-line Active Galactic Nuclei (AGNs) spanning redshifts between 0.15 and 3.87 with a median value of 1.68. The optical-to-X-ray spectral index of the broad-line AGNs is 1.47, typical of optically-selected Type I quasars and is found to correlate with the rest frame X-ray and optical monochromatic luminosities at 2 keV and 2500 angstroms respectively. Consistent with previous studies, we find alpha_ox not to be correlated with z. In addition, 32 and 116 X-ray sources are, respectively absorption and emission-line galaxies at z<0.76. From a line ratio diagnostic diagram it is found that in about 50% of these emission line galaxies, the emission lines are powered significantly by the AGN. Thirty of the XMM sources are detected at one or more radio frequencies. In addition, 24 sources have ambiguous identification: in 8 cases, two XMM sources have a single optical source within 6 arcsecs of each of them, whereas, 2 and 14 XMM sources have, respectively, 3 and 2 possible optical sources within 6 arcsecs of each of them.