Gemini near-infrared spectroscopy of luminous z ∼ 6 quasars: Chemical abundances, black hole masses, and Mg II absorption

We present Gemini near-infrared spectroscopic observations of six luminous quasars at z = 5.8 to ∼6.3. Five of them were observed using Gemini South GNIRS, which provides a simultaneous wavelength coverage of 0.9-2.5 μm in cross-dispersion mode. The other source was observed in the 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 ∼4 Z_⊙, and a comparison with low-redshift observations shows no strong evolution in metallicity up to z ∼ 6. The Fe II/Mg II ratio of the quasars is 4.9 ± 1.4, consistent with low-redshift measurements. We estimate central black hole masses of 10⁹-10¹⁰ M _⊙ and Eddington luminosity ratios of order unity. We identify two Mg II λλ2796, 2803 absorbers with rest equivalent width W ₀^λ2796 > 1 Å at 2.2 < z < 3 and three Mg II absorbers with W₀^λ2796 > 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 W₀^λ2796 > 1.5 Å are consistent with no cosmic evolution up to z > 4.