Gas metallicity in the narrow-line regions of high-redshift active galactic nuclei

¹ INAF - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy e-mail: [tohru;maiolino;marconi]@arcetri.astro.it
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 151-8588, Japan

Received: 30 August 2005
Accepted: 4 October 2005

Abstract

We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs at redshift found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift to investigate the gas metallicity of the narrow-line regions and their evolution in this redshift range. Such spectra are also compared with UV spectra of local Seyfert 2 galaxies. The observational data are inconsistent with the predictions of shock models, suggesting that the narrow-line regions are mainly photoionized. The photoionization models with dust grains predict line flux ratios which are also in disagreement with most of the observed values, suggesting that the high-ionization part of the narrow-line regions (which is sampled by the available spectra) is dust-free. The photoionization dust-free models provide two possible scenarios which are consistent with the observed data: low-density gas clouds ( cm^-3) with a sub-solar metallicity (), or high-density gas clouds ( cm^-3) with a wide range of gas metallicity (). Regardless of the specific interpretation, the observational data do not show any evidence for a significant evolution of the gas metallicity in the narrow-line regions within the redshift range . Instead, we find a trend for more luminous active galactic nuclei to have more metal-rich gas clouds (luminosity-metallicity relation), which is in agreement with the same finding in the studies of the broad-line regions. The lack of evolution for the gas metallicity of the narrow-line regions implies that the major epoch of star formation in the host galaxies of these active galactic nuclei is at .