The evolution of the broad-line region among SDSS quasars

Free access article

Issue		A&A Volume 447, Number 1, February III 2006


Page(s)		157 - 172
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054024

A&A 447, 157-172 (2006)
DOI: 10.1051/0004-6361:20054024

The evolution of the broad-line region among SDSS quasars

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

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

(Received 9 August 2005 / Accepted 6 October 2005 )

Abstract
Based on 5344 quasar spectra taken from the SDSS Data Release 2, the dependences of various emission-line flux ratios on redshift and quasar luminosity are investigated in the ranges $2.0 \leq z \leq 4.5$ and $-24.5 \geq M_B \geq -29.5$ . We show that the emission lines in the composite spectra are fitted better with power-law profiles than with double Gaussian or modified Lorentzian profiles, and in particular we show that the power-law profiles are more appropriate to measure broad emission-line fluxes than other methods. The composite spectra show that there are statistically significant correlations between quasar luminosity and various emission-line flux ratios, such as NV/CIV and NV/HeII, while there are only marginal correlations between quasar redshift and emission-line flux ratios. We obtain detailed photoionization models to interpret the observed line ratios. The correlation of line ratios with luminosity is interpreted in terms of higher gas metallicity in more luminous quasars. For a given quasar luminosity, there is no metallicity evolution for the redshift range $2.0 \leq z \leq 4.5$ . The typical metallicity of BLR gas clouds is estimated to be $Z \sim 5~ Z_\odot$ , although the inferred metallicity depends on the assumed BLR cloud properties, such as their density distribution function and their radial distribution. The absence of a metallicity evolution up to $z \sim 4.5$ implies that the active star-formation epoch of quasar host galaxies occurred at $z \ga 7$ .

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

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