2006A&A...453..525N

We combine emission line and X-ray luminosities for 45 sources from the Chandra Deep Field South (CDF-S), and seven HELLAS sources, to obtain a new sample of 52 X-ray selected type-II active galactic nuclei (AGNs). Eighteen of our sources are very luminous with a typical, absorption-corrected 2-10keV luminosity of fewx10⁴⁴erg/s (type-II QSOs). We compare the emission line properties of the new sources with emission line and X-ray luminosities of known low redshift, mostly lower luminosity AGNs by using a composite spectrum. We find that L_[OIII]/L_2–10 and L_[OII]/L_2–10 decrease with L(2-10keV) such that L_[OIII]/L_2–10∝L_2–10^–0.42. The trend was already evident, yet neglected in past low redshift samples. This lead to erroneous calibration of the line-to-X-ray luminosity in earlier AGN samples. The analysis of several type-I samples shows the same trend with a similar slope but a median L_[OIII]/L_2–10 which is larger by a factor of about two compared with optically selected type-II samples. We interpret this shift as due to additional reddening in type-II sources and comment in general on the very large extinction in many type-II objects and the significantly smaller average reddening of the SDSS type-II AGNs. The decrease of L_[OIII]/L_2–10 with L(2-10keV) is large enough to suggest that a significant fraction of high luminosity high redshift type-II AGNs have very weak emission lines that may have escaped detection in large samples. A related decrease of EW([OIII]λ5007) with optical continuum luminosity is demonstrated by an analysis of 12000 type-I SDSS AGNs. The new correlations found here are important for deriving accurate luminosity functions for AGNs and their neglect may explain past discrepancies between emission line and X-ray samples.