Metallicity in Quasar Broad Line Regions at Redshift $\sim$ 6
  Broad line regions (BLRs) in high-redshift quasars provide crucial
information of chemical enrichment in the early universe. Here we present a
study of BLR metallicities in 33 quasars at redshift $5.7<z<6.4$. Using the
near-IR spectra of the quasars obtained from the Gemini telescope, we measure
their rest-frame UV emission line flux and calculate flux ratios. We then
estimate BLR metallicities with empirical calibrations based on photoionization
models. The inferred median metallicity of our sample is a few times the solar
value, indicating that the BLR gas had been highly metal-enriched at $z\sim6$.
We compare our sample with a low-redshift quasar sample with similar
luminosities and find no evidence of redshift evolution in quasar BLR
metallicities. This is consistent with previous studies. The Fe II$/$Mg II flux
ratio, a proxy for the Fe$/\alpha$ element abundance ratio, shows no redshift
evolution as well, further supporting rapid nuclear star formation at $z\sim6$.
We also find that the black hole mass-BLR metallicity relation at $z\sim6$ is
consistent with the relation measured at $2<z<5$, suggesting that our results
are not biased by a selection effect due to this relation.
