Program: GN-2018A-FT-112

Quasars at high redshift (z > 6) are an important probe of the distant universe, for understanding the origin and progress of cosmic reionization, the early growth of supermassive black holes (SMBHs), and the evolution of the host galaxies. By exploiting the exquisite imaging data produced by the Subaru Hyper Suprime-Cam (HSC) survey, we have been carrying out a spectroscopic survey for high-z quasars, and have discovered 80 new quasars at 5.8 < z < 6.9 so far. On the other hand, there have been only two quasars known at z > 7, hampering statistical studies of quasars and SMBHs at such high redshift. These two z > 7 quasars are extremely luminous (apparent magnitudes J ~ 20 AB mag) and have large SMBH masses (~10^9 Msun). Very recently (April 2018), we have discovered a new quasar at z > 7 from the HSC data. This quasar has an estimated near-IR magnitude of J ~ 23 AB mag, and likely represents a unique population of SMBHs with much lower mass and/or mass-accretion efficiency than the other two z > 7 quasars. Here we propose GNIRS observations of this new quasar, in order to measure the CIV 1549 and MgII 2800 lines in the near-IR spectrum. We aim to determine the accurate redshift and SMBH mass of this precious object, and to obtain critical clues as to the earliest stage of the SMBH evolution in the extremely distant Universe.