Issue |
A&A
Volume 527, March 2011
|
|
---|---|---|
Article Number | A100 | |
Number of page(s) | 15 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201015584 | |
Published online | 02 February 2011 |
The mass-metallicity relation of SDSS quasars
1
Graduate School of Science and Engineering, Ehime University,
2-5 Bunkyo-cho, 790-8577,
Matsuyama,
Japan
e-mail: kenta@cosmos.phys.sci.ehime-u.ac.jp
2
Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho,
790-8577
Matsuyama,
Japan
3
Optical and Infrared Astronomy Division, National Astronomical
Observatory of Japan, 2-21-1 Osawa,
181-8588
Mitaka,
Japan
4
Dipartimento di Fisica e Astronomia, Universita degli Studi di
Firenze, Largo E. Fermi
2, 50125
Firenze,
Italy
5
INAF – Osservatorio Astrofisico di Roma, via di Frascati 33, 00040
Monte Porzio Catone,
Italy
Received:
13
August
2010
Accepted:
19
November
2010
Active galactic nuclei (AGNs) are characterized by a clear correlation between luminosity and metallicity (LAGN-ZAGN relation). The origin of this correlation is not clear. It may result from a relation between the black hole mass (MBH) and metallicity, or from a relation between the accretion rate (L/LEdd) and metallicity. To investigate the origin of the LAGN-ZAGN relation, we use optical spectra of 2383 quasars at 2.3 < z < 3.0 from the Sloan Digital Sky Survey. By using this data set, we constructed composite spectra of 33 subsamples in intervals of both MBH and L/LEdd. From these composite spectra we measured emission-line flux ratios that are sensitive to the metallicity of the broad line region (BLR): specifically, N vλ1240/C ivλ1549, N vλ1240/He iiλ1640, (Si ivλ1398+O iv]λ1402)/C ivλ1549, and Al iiiλ1857/C ivλ1549. We find that there is a significant correlation between MBH and ZBLR as inferred from all four metallicity-sensitive emission-line flux ratios. This result strongly suggests that the observed LAGN-ZAGN relation is mostly a consequence of the MBH-ZAGN relation. The relation between MBH and ZBLR is likely a consequence of both the MBH-Mbul relation and of the mass-metallicity relation in the host galaxy. We also find that L/LEdd correlates with the emission line flux ratios involving N v (more specifically, N v/C iv and N v/He ii), while it does not correlate with the other two metallicity sensitive emission line flux ratios, i.e., (Si iv+O iv])/C iv and Al iii/C iv. These correlations indicate that the emission-line flux ratios involving N v depend on both the metallicity and relative abundance of nitrogen. We suggest that the relation between L/LEdd and those line ratios involving nitrogen is caused by a delay of the black hole accretion rate relative to the onset of nuclear star formation of about 108 years, which is the timescale required for the nitrogen enrichment.
Key words: galaxies: active / galaxies: evolution / galaxies: nuclei / quasars: emission lines / quasars: general
© ESO, 2011
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