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_odot, 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_odot and Eddington luminosity ratios of order unity. We identify two Mg II λλ2796, 2803 absorbers with rest equivalent width W > 1 Å at 2.2 < z < 3 and three Mg II absorbers with W > 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 > 1.5 Å are consistent with no cosmic evolution up to z > 4.

Based on observations obtained at the Gemini Observatory (acquired through the Gemini Science Archive), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).