The mass-metallicity relation of SDSS quasars

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