Large-scale structure formation and the physical processes governing galaxy evolution in high density environments are important open issues. Direct look-back studies of the progenitors of galaxy clusters enable to address these questions. The unprecedented combination of area and depth of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) allows us, for the first time, to study protoclusters systematically even at high redshifts. Based on HSC-SSP, augmented with u-band imaging from CLAUDS, we constructed a large sample of protocluster candidates at z∼3−5 in the ∼26 deg2 Deep layer. We find that brighter dropout galaxies are more richly populating the protocluster candidates than coeval field environments, a trend which persists at all redshifts from z∼3 to z∼5. We now propose follow-up spectroscopy for three protocluster candidates at z∼3 in order to unveil the physical mechanisms causing such an environmental difference of galaxy properties. Our proposed FOCAS observations will reveal the relation between the positions of bright galaxies and internal structure of protoclusters. Combining the physical properties of protocluster members derived by multi-wavelength data, we will distill the environmental impact on early galaxy evolution in protoclusters at z∼3.
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