Abstract

Our on-going z~7 quasar survey has yielded more than 35 new luminous quasars at 6.3 < z <= 7.6, which forms the first large statistical sample of reionization-era quasars. I will present our recent works related to the intergalactic medium (IGM) evolution in the reionization epoch and the early supermassive black hole (SMBH) growth using this new quasar sample. We measure the Lyman-alpha and Lyman-beta effective optical depth at 5.3<z<6.5 using both individual sightlines and the stacked spectrum. Combining our new measurements and previous results, we obtain a best-fit evolution for the Lyman-alpha optical depth at z~5.3-6.1. We also build a large high redshift transmission spike sample including more than 400 Lyman-alpha or Lyman-beta transmission spikes at 5.5<z<6.3. The existence of these transmission spikes at redshift up to z~6.3 and its evolution suggests a rapid transition phase of IGM neutral fraction from z~7 to z~6.3. The study of damping wing features from our newly discovered z=7.5 quasar and the other z=7.0 quasar indicates a large scatter of the IGM neutral fraction from z=7.5 to z=7.0, indicative of a patchy reionization process. In addition, with data from the on-going near-infrared spectroscopic survey, we measure the BH mass and Eddington ratio, and investigate the properties of quasar broad emission lines. These quasars set new challenges to the formation and growth history of the earliest SMBHs. In particular, our newly discovered z=7.5 and 7.6 quasars hosting billion solar mass black holes require the most massive seed black holes compared to any other known quasars. The result is more consistent with a massive seed black hole model (e.g., direct collapse).