The rest-frame UV luminosity function of protocluster galaxies at z~4 revealed by HSC-SSP

Various studies report the higher star formation rate of protocluster galaxies at z~2. It is crucial to investigate the picture at higher redshift. On the other hand, it was not impractical due to the lack of enough samples of the protoclusters. We estimate the rest-frame ultraviolet luminosity function of g-dropout galaxies in 177 protocluster candidates (PC UVLF) at z∼4 selected in the PDR1 of the Hyper Suprime-Cam Subaru Strategic Program. Comparing it with the UVLF of field galaxies at the same redshift, we find that the PC UVLF shows a significant excess toward the bright end. This study shows that the enhancement in star formation in overdense regions can generally be seen as early as at z∼4. We also estimate the star formation rate density in protocluster regions as 6-20% of the cosmic SFRD, based on the measured PC UVLF after correction for the selection incompleteness in our protocluster sample. This high value suggests that protoclusters make a non-negligible contribution to the cosmic SFRD at z∼4, as previously suggested by simulations. We will also show the result of the latest HSC-SSP data release.