The correlation of narrow line emission and X-ray luminosity in active galactic nuclei

Aims. 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 2-10 keV luminosity of few " 10 44 erg s # 1 (type-II QSOs). Methods. 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. Results. We find that L(OIII)/L2# 10 and L(OII)/L2# 10 decrease with L(2-10 keV) such that L(OIII)/L2# 10 $ L # 0.42 2# 10 . 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)/L2# 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)/L2# 10 with L(2-10 keV) 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((O!!! )! 5007) 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.