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
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.
Key words: galaxies: active / galaxies: Seyfert / quasars: emission lines / X-rays: galaxies
© ESO, 2006