Obscured quasars in the epoch of reionization (EoR) are a key population to reveal the initial growth of supermassive black holes (SMBHs). While very few candidates have been reported, models and observations suggest that such objects are prevalent in the high-z (z ≥ 6) universe, and that the ∼300 UV-luminous quasars known today represent just a small portion of early SMBH growth. Recently, the HSC-SSP survey has serendipitously discovered a new population of galaxies at z ∼ 6 showing extremely luminous Lyα emission, with clearly distinct properties from any known types of objects in the EoR. The presence of obscured quasar activity has since been confirmed with JWST and other spectroscopy. Here we propose a statistical exploration of this high-z obscured quasar population, with FOCAS spectroscopy of HSC-SSP candidates including extended sources. Our immediate objective is to establish the first complete sample of obscured quasars at z = 6. By comparing with the statistics of unobscured quasars and galaxies, we aim to address the following fundamental questions; (i) How much SMBH growth is obscured in the EoR?; (ii) What types and fraction of EoR galaxies host quasars? The answers brought by this program will provide critical constraints on models of early SMBH evolution.
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