Program: GS-2019B-Q-135

Quasars at reionization provide direct probes of the evolution of supermassive black holes (SMBHs), the cosmic reionization history, and the build-up of massive galaxies at cosmic dawn. In the past two years, we have discovered 32 new luminous quasars at z >6.5. This doubles the number of known quasars at this redshift, forming the largest uniformly selected luminous quasar sample at this epoch. Here, we propose to use Gemini/GMOS-S to obtain deep optical spectroscopic observations of four newly discovered quasars in the southern sky that do not yet have high quality spectra. The GMOS observations will map the reionization history at the crucial redshift range of 6.5<z<7.0, when the IGM neutral fraction increases drastically towards higher redshift. We will measure or tightly constrain the Ly-alpha/Ly-beta optical depth at z>6.3 for the first time, and search for the highest redshift Ly-alpha and Ly-beta transmission spikes which provide a more sensitive test to the neutral hydrogen fraction evolution during reionization. Furthermore, these optical spectra will allow detailed modeling of quasar Ly-alpha region, including the profile of Gunn-Peterson damping wing and the size of proximity zone, providing new constraints on IGM ionization, quasar life time and SMBH growth.