The Evolution of Chemical Abundance in Quasar Broad Line Region
  We study the relation between the metallicity of quasar broad line region
(BLR) and black hole (BH) mass ($10^{7.5}M_{\odot} \sim 10^{10}M_{\odot}$) and
quasar bolometric luminosity ($10^{44.6}erg/s \sim 10^{48} erg/s$) using a
sample of $\sim$130,000 quasars at $2.25\leq z\leq 5.25$ from Sloan Digital Sky
Survey (SDSS) Data Release 12 (DR12). We generate composite spectra by stacking
individual spectra in the same BH mass (bolometric luminosity) and redshift
bins and then estimate the metallicity of quasar BLR using
metallicity-sensitive broad emission-line flux ratios based on the
photoionization models. We find a significant correlation between quasar BLR
metallicity and BH mass (bolometric luminosity) but no correlation between
quasar BLR metallicity and redshift. We also compare the metallicity of quasar
BLR and that of host galaxies inferred from the mass-metallicity relation of
star-forming galaxy at $z\sim2.3$ and $z\sim3.5$. We find quasar BLR
metallicity is 0.3 $\sim$ 1.0 dex higher than their host galaxies. This
discrepancy cannot be interpreted by the uncertainty due to different
metallicity diagnostic methods, mass-metallicity relation of galaxy,
metallicity gradient in quasar host galaxies, BH mass estimation, the effect of
different spectral energy distribution (SED) models, and a few other potential
sources of uncertainties. We propose a possibility that the high metallicity in
quasar BLR might be caused by metal enrichment from massive star formation in
the nucleus region of quasars or even the accretion disk.
