It is known that X-ray selected sources are dominated by AGN which distributed in a wide redshift range, up to z ~ 4. However it is not easy to obtain the spectroscopic redshifts for all sources of the wide field due to the strong competition of different follow-ups programs on the appropriate instruments and faintness of the optical counterparts (an influence of the extragalactic absorption). The multiwavelength information of the survey can be effectively used for building of the observational Spectral Energy Distribution (SEDs) for each source. Then, using the fiffing technique it is possible to compare this SED with an observational or syntetic template and compute so called a photometric redshift of given object. The calculation of the photometric redshifts is very effective for the normal galaxies (Ilbert et al. 2006) but not so accurate for the AGN (Salvato et al. 2009). However for the moment it is the only way to have the redshift information in deep surveys, except spectroscopic redshifts which are still preferable. The photometric redshifts are very helpful in the studies of the evolution effects (dependence the physical properties of sources on redshift) but uncertainties in the photometric redshift measurements are too high to use them for the Large Scale Structure studies.
The XMM-LSS field contains 5142 X-ray sources with optical (and multiwavelength) couterparts and for only 783 of them have the spectroscopic redshifts. Therefore we calculated the photo-z for the rest of the sources where it was possible. The calculation was performed using 11 bands: u, g, r, i, z (CFHTLS); J and K (UKIDSS); 3.6 and 4.5 μm (Spitzer/IRAC); and far-UV and near-UV bands of GALEX with the public codeLePhare and Salvato et al. (2009) templates. The datails of the photometric redshift calculation is presented in our paper Melnyk et al. (2013) and the catalog is available online. Fig. 9 presents the relation between the photometric and spectroscopic redshifts for extended objects (ext: we assume that this sample mostly consists of galaxy dominated objects), and point-like objects (ptl: AGN/ QSO dominated sample).