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

Free access article

Issue		A&A Volume 447, Number 3, March I 2006


Page(s)		863 - 876
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054127

A&A 447, 863-876 (2006)
DOI: 10.1051/0004-6361:20054127

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

T. Nagao^{1, 2}, R. Maiolino¹ and A. Marconi¹

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 $1.5 \la z \la 3.7$ found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift $1.2 \la z \la 3.8$ 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 ( $n_{\rm H} \la 10^3$ cm^-3) with a sub-solar metallicity ( $0.2 \la Z_{\rm gas}/Z_\odot \la 1.0$ ), or high-density gas clouds ( $n_{\rm H} \sim 10^5$ cm^-3) with a wide range of gas metallicity ( $0.2 \la Z_{\rm gas}/Z_\odot \la 5.0$ ). 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 $1.2 \la z \la 3.8$ . 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 $z \ga 4$ .

Key words: galaxies: active -- galaxies: evolution -- galaxies: nuclei -- quasars: emission lines -- quasars: general

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