Program: GN-2016B-Q-39

Understanding how the neutral dark Universe was re-ionized by the first stars, galaxies and black holes is one of the key goals of the modern observational cosmology. Bright UV continuum of high-redshift QSOs can be used as a background light to probe this cosmic re-ionization since it is absorbed by neutral hydrogen (Gunn-Peterson test). However, such luminous QSOs are rare, and difficult to be found. To find such luminous QSOs at z>6.5, one needs a large survey of 1000s of deg$^2$ in near-infrared. In the last semester, we successfully found a new luminous QSO at z=6.6, as a result of many years of team work. This QSO is the 2nd most luminous at z>6.6, and thus, provides us a bright light to shine the distant Universe. We propose a deep optical and near-infrared spectroscopy of this QSO, in order to (a) To perform the Gunn-Peterson and other absorption tests to constrain reionization history of the Universe at one of the highest redshifts. (b) To reveal accurate redshift, black hole mass, and chemical abundances of the QSO, and IGM at z$\sim$6.6. As described in the text, some of these measurements are to be performed for the first time at z$\sim$6.6.