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