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Broad line regions (BLRs) in high-redshift quasars provide crucialinformation of chemical enrichment in the early universe. Here we present astudy of BLR metallicities in 33 quasars at redshift $5.7<z<6.4$. Using thenear-IR spectra of the quasars obtained from the Gemini telescope, we measuretheir rest-frame UV emission line flux and calculate flux ratios. We thenestimate BLR metallicities with empirical calibrations based on photoionizationmodels. The inferred median metallicity of our sample is a few times the solarvalue, indicating that the BLR gas had been highly metal-enriched at $z\sim6$.We compare our sample with a low-redshift quasar sample with similarluminosities and find no evidence of redshift evolution in quasar BLRmetallicities. This is consistent with previous studies. The Fe II$/$Mg II fluxratio, a proxy for the Fe$/\alpha$ element abundance ratio, shows no redshiftevolution as well, further supporting rapid nuclear star formation at $z\sim6$.We also find that the black hole mass-BLR metallicity relation at $z\sim6$ isconsistent with the relation measured at $2<z<5$, suggesting that our resultsare not biased by a selection effect due to this relation.

Metallicity in Quasar Broad Line Regions at Redshift $\sim$ 6