On the use of photometric redshifts for X-ray selected AGNs

In this paper we present photometric redshift estimates for a sample of X-ray selected sources detected in the wide-field ( 2 deg^2), bright [ f_X(0.5{-}8 keV)≈10^-14 erg s^-1 cm^-2] XMM-Newton/2dF survey. Unlike deeper X-ray samples comprising a large fraction of sources with colours dominated by the host galaxy, our bright survey primarily probes the QSO X-ray population. Therefore photometric redshift methods employing both galaxy and QSO templates need to be used. We employ the photometric redshift technique of Hatziminaoglou et al. (2000) using 5-band photometry from the SDSS. We separate our X-ray sources according to their optical profiles into point-like and extended. We apply QSO and galaxy templates to the point-like and extended sources respectively. X-ray sources associated with Galactic stars are identified and discarded from our sample on the basis of their unresolved optical light profile, their low X-ray-to-optical flux ratio and their broad-band colours that are best fit by stellar templates. Comparison of our results with spectroscopic redshifts available allows calibration of our method and estimation of the photometric redshift accuracy. For 70 per cent of the point-like sources photometric redshifts are correct within δ z ⪉ 0.3 (or 75 per cent have δ z/(1+z) ⪉ 0.2), and the rms scatter is estimated to be σ_z= 0.30. Also, in our X-ray selected point-like sample we find that about 7 per cent of the sources have optical colours redder than those of optically selected QSOs. Photometric redshifts for these systems using existing QSO templates are most likely problematic. For the optically extended objects the photometric redshifts work only in the case of red (g - r > 0.5 mag) sources yielding δ z ⪉ 0.15 and δ z/(1+z) ⪉ 0.2 for 73 and 93 per cent respectively. The results above are consistent with earlier findings from the application of combined galaxy/QSO photometric redshift techniques in the Chandra Deep Field North. However, we find that the above photometric redshift technique does not work in the case of extended sources with blue colours (g-r<0.5). Although these form a significant fraction of the extended sources (≈40%), they cannot be fit successfully by QSO or galaxy templates, or any linear combination of the two.