Issue |
A&A
Volume 447, Number 1, February III 2006
|
|
---|---|---|
Page(s) | 157 - 172 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20054024 | |
Published online | 27 January 2006 |
The evolution of the broad-line region among SDSS quasars
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
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
and
.
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
. The typical metallicity of BLR gas clouds
is estimated to be
, 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
implies that the active star-formation epoch of quasar host
galaxies occurred at
.
Key words: galaxies: active / galaxies: evolution / galaxies: nuclei / galaxies: quasars: emission lines / galaxies: quasars: general
© ESO, 2006
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