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

¹ School of Physics and Astronomy and the Wise Observatory, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel e-mail: netzer@wise.tau.ac.il
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany

Accepted: 14 September 2005

Abstract

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  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 / and / decrease with L(2–10 keV) such that / . 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 / 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 / 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] ) 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.