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

TL;DR

This study analyzes the metallicity of broad line regions in 33 quasars at redshift 5.7 to 6.4, revealing highly enriched gas and no significant evolution compared to lower redshifts, indicating rapid early universe star formation.

Contribution

First measurement of BLR metallicities in a large sample of high-redshift quasars using near-IR spectra and empirical calibrations, showing early metal enrichment and no redshift evolution.

Findings

Median BLR metallicity is a few times solar at z~6.

No evidence of redshift evolution in BLR metallicity.

Fe II/Mg II ratio remains constant, indicating rapid star formation.

Abstract

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.