2020ApJ...898..105O

We present deep (9 hr) Gemini-N/Gemini Near-InfraRed Spectrograph near-infrared spectroscopic observations of ULAS J1342+0928, a luminous quasar at z = 7.54. Various broad emission lines were detected, as well as the underlying continuum and iron forests over the rest-frame wavelength 970-2930 Å. There is a clear trend that higher-ionization emission lines show larger blueshifts with C IV λ1549 exhibiting 5510_–110⁺²⁴⁰ km s^–1 blueshift with respect to the systematic redshift from the far-infrared [C II] 158µm emission line. Those high-ionization lines have wide profiles with FWHM more than 10,000 km s^–1. A modest blueshift of 340_–80⁺¹¹⁰ km s^–1 is also seen in Mg II, the lowest-ionization line identified in the spectrum. The updated Mg II-based black hole mass of M_BH=9.1_–1.3^+1.4×10⁸M_☉ and the Eddington ratio of L_bol/L_Edd=1.1_–0.2^+0.2 confirm that ULAS J1342+0928 is powered by a massive and actively accreting black hole. There is no significant difference in the emission-line ratios such as Si IV/C IV and Al III/C IV when compared to lower-redshift quasars in a similar luminosity range, which suggests early metal pollution of the broad-line-region clouds. This trend also holds for the Fe II/Mg II line ratio, known as a cosmic clock that traces the iron enrichment in the early universe. Different iron templates and continuum fitting ranges were used to explore how the Fe II/Mg II measurement changes as a function of spectral modeling. Quasars at even higher redshift or at fainter luminosity range (L_bol≲10⁴⁶ erg s^–1) are needed to probe the sites of early metal enrichment and a corresponding change in the Fe II/Mg II ratio.