2021ApJ...923..262Y

We report the results of near-infrared spectroscopic observations of 37 quasars in the redshift range 6.3 < z <= 7.64, including 32 quasars at z > 6.5, forming the largest quasar near-infrared spectral sample at this redshift. The spectra, taken with Keck, Gemini, VLT, and Magellan, allow investigations of central black hole mass and quasar rest-frame ultraviolet spectral properties. The black hole masses derived from the Mg II emission lines are in the range (0.3-3.6) x 10⁹ M_☉, which requires massive seed black holes with masses >=10³-10⁴ M_☉, assuming Eddington accretion since z = 30. The Eddington ratio distribution peaks at λ_Edd ∼ 0.8 and has a mean of 1.08, suggesting high accretion rates for these quasars. The C IV-Mg II emission-line velocity differences in our sample show an increase of C IV blueshift toward higher redshift, but the evolutionary trend observed from this sample is weaker than the previous results from smaller samples at similar redshift. The Fe II/Mg II flux ratios derived for these quasars up to z = 7.6, compared with previous measurements at different redshifts, do not show any evidence of strong redshift evolution, suggesting metal-enriched environments in these quasars. Using this quasar sample, we create a quasar composite spectrum for z > 6.5 quasars and find no significant redshift evolution of quasar broad emission lines and continuum slope, except for a blueshift of the C IV line. Our sample yields a strong broad absorption line quasar fraction of ∼24%, higher than the fractions in lower-redshift quasar samples, although this could be affected by small sample statistics and selection effects.