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

Abstract

Active galactic nuclei (AGNs) are characterized by a clear correlation between luminosity and metallicity (L_AGN-Z_AGN relation). The origin of this correlation is not clear. It may result from a relation between the black hole mass (M_BH) and metallicity, or from a relation between the accretion rate (L/L_Edd) and metallicity. To investigate the origin of the L_AGN-Z_AGN 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 M_BH and L/L_Edd. 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 M_BH and Z_BLR as inferred from all four metallicity-sensitive emission-line flux ratios. This result strongly suggests that the observed L_AGN-Z_AGN relation is mostly a consequence of the M_BH-Z_AGN relation. The relation between M_BH and Z_BLR is likely a consequence of both the M_BH-M_bul relation and of the mass-metallicity relation in the host galaxy. We also find that L/L_Edd 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/L_Edd 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.