No Redshift Evolution in the Broad-line-region Metallicity up to z=7.54: Deep Near-infrared Spectroscopy of ULAS J1342+0928

Abstract

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 angstrom. There is a clear trend that higher-ionization emission lines show larger blueshifts with C IV lambda 1549 exhibiting 5510(-110)(+240) km s(-1) blueshift with respect to the systematic redshift from the far-infrared [C II] 158 mu m 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) x 10(8) M-circle dot 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 less than or similar to 10(46) erg s(-1)) are needed to probe the sites of early metal enrichment and a corresponding change in the Fe II/Mg II ratio.