We present Gemini near-infrared spectroscopic observations of six luminous quasars at z = 5.8 \sim 6.3 .
Five of them were observed using Gemini-South/GNIRS , which provides a simultaneous wavelength coverage of 0.9–2.5 \mu m in cross dispersion mode .
The other source was observed in K band with Gemini-North/NIRI .
We calculate line strengths for all detected emission lines and use their ratios to estimate gas metallicity in the broad-line regions of the quasars .
The metallicity is found to be supersolar with a typical value of \sim 4 Z _ { \sun } , and a comparison with low-redshift observations shows no strong evolution in metallicity up to z \sim 6 .
The Fe ii /Mg ii ratio of the quasars is 4.9 \pm 1.4 , consistent with low-redshift measurements .
We estimate central BH masses of 10 ^ { 9 } to 10 ^ { 10 } M _ { \sun } and Eddington luminosity ratios of order unity .
We identify two Mg ii \lambda \lambda 2796,2803 absorbers with rest equivalent width W _ { 0 } ^ { \lambda 2796 } > 1 Å at 2.2 < z < 3 and three Mg ii absorbers with W _ { 0 } ^ { \lambda 2796 } > 1.5 Å at z > 3 in the spectra , with the two most distant absorbers at z = 4.8668 and 4.8823 , respectively .
The redshift number densities ( dN / dz ) of Mg ii absorbers with W _ { 0 } ^ { \lambda 2796 } > 1.5 Å are consistent with no cosmic evolution up to z > 4 .