Context :

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 \times 10 ^ { 44 } ergs 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 ] } } / { L _ { 2 - 10 } } and { L _ { [ OII ] } } / { L _ { 2 - 10 } } decrease with L ( 2-10 keV ) such that { L _ { [ OIII ] } } / { L _ { 2 - 10 } } \propto { 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 ( 2–10 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 iii ] \lambda 5007 ) with optical continuum luminosity is demonstrated by an analysis of 12,000 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 .

Conclusions :