Ting-Yi Lu (NTHU), Tomotsugu Goto (NTHU), Tetsuya Hashimoto (NTHU), Daryl Joe D. Santos (NTHU), Seong Jin Kim (NTHU), Alvina Y. L. On (UCL), Yoshiki Matsuoka (Ehime U.), Toru Nagao (Ehime U.), and the SHELLQs team
It is crucial to observationally constrain the ionization history of the intergalactic medium (IGM) during the epoch of reionization (EoR) because it is strongly connected to the structure formation history and the emission mechanisms of ionizing sources. However, such an important topic has yet to be resolved due to the limited sample size at high redshifts. Quasars (QSOs) at z>6 are one of the best probes of the IGM ionization state during the EoR because they are luminous enough to be detected. Nevertheless, only ~100 luminous QSOs at z>6 are found over twenty years of surveys. A majority of theoretical works on EoR made use of constraints from this small QSOs sample such as the IGM neutral fraction and the HI photoionization rate. Furthermore, the current bright QSO sample may be biased towards overdense regions, which may potentially result in misleading values of the IGM neutral fraction. To overcome these problems, we use the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs), which allows the sample to be extended towards fainter (less biased) QSOs with better statistics. With 93 faint SHELLQs QSOs at z>5.5, we present the latest observational constraints on the HI photoionization rate, neutral fraction, opacity fluctuation of IGM, and the redshift evolution of the QSO HII regions.