Chemical Abundance of z~6 Quasar Broad-Line Regions in the XQR-30 Sample

TL;DR

This study investigates the chemical composition of quasar broad-line regions at redshifts around 6, revealing high metallicities and no significant evolution in emission-line ratios compared to lower redshifts, using UV spectra from VLT/X-shooter and Gemini/GNIRS.

Contribution

It provides the first comprehensive analysis of metallicity-sensitive emission lines in high-redshift quasars, showing consistent high metallicities and no evolution with redshift, and clarifies the impact of CIV blueshift on line ratios.

Findings

High metallicity (2-4 times solar) in z~6 quasar broad-line regions.

No significant evolution in emission-line ratios from z~2 to z~6.

CIV blueshift correlates with equivalent width, affecting line ratios.

Abstract

The elemental abundances in the broad-line regions of high-redshift quasars trace the chemical evolution in the nuclear regions of massive galaxies in the early universe. In this work, we study metallicity-sensitive broad emission-line flux ratios in rest-frame UV spectra of 25 high-redshift (5.8 < z < 7.5) quasars observed with the VLT/X-shooter and Gemini/GNIRS instruments, ranging over $\log(M_{\rm{BH}}/M_{\odot})= 8.4-9.8$ in black hole mass and $\log(L_{\rm{bol}}/\rm{erg\, s}^{-1})= 46.7-47.7$ in bolometric luminosity. We fit individual spectra and composites generated by binning across quasar properties: bolometric luminosity, black hole mass, and blueshift of the \civ\, line, finding no redshift evolution in the emission-line ratios by comparing our high-redshift quasars to lower-redshift (2.0 < z < 5.0) results presented in the literature. Using Cloudy-based locally optimally-emitting cloud photoionisation model relations between metallicity and emission-line flux ratios, we find the observable properties of the broad emission lines to be consistent with emission from gas clouds with metallicity that are at least 2-4 times solar. Our high-redshift measurements also confirm that the blueshift of the CIV emission line is correlated with its equivalent width, which influences line ratios normalised against CIV. When accounting for the CIV blueshift, we find that the rest-frame UV emission-line flux ratios do not correlate appreciably with the black hole mass or bolometric luminosity.