2019ApJ...882..144K

We present near-infrared 1.1-1.3 and 1.3-1.6 µm spectra of the Lyα-luminous quasar PSO J006.1240+39.2219 at z = 6.617 ± 0.003 obtained with the NIRSPEC spectrograph at the Keck II telescope. The spectra cover the C IV λ1549, C III] λ1909 emission lines and part of the UV continuum of the quasar. From the NIRSPEC observations of PSO J006.1240+39.2219, we constrain the spectral slope of its UV continuum to be α_λ = -1.35 ± 0.26 and measure an absolute magnitude of M₁₄₅₀ = -25.60. Using the scaling relation between black hole mass, width of the C IV line and ultraviolet continuum luminosity, we derive a black hole mass of (2.19 ± 0.30) x 10⁸M_☉, which is consistent but somewhat smaller than the typical black hole masses of z >= 6 quasars of similar luminosities. The inferred accretion rate of L_Bol/L_Edd >= 2 indicates that PSO J006.1240+39.2219 is in the phase of the rapid growth of its supermassive black hole characterized by the high N V/C IV line ratio, N V/C IV > 1, and the lower level of ionization of its circumnuclear gas than in other high-redshift luminous quasars. The N V/C IV line ratio of PSO J006.1240+39.2219 implies a relatively high abundance of nitrogen in its circumnuclear gas. This abundance might be produced by the post-starburst population of stars that provide the fuel for black hole accretion.