High-z radio galaxies (HzRGs) are useful probes of the most overdense regions which shed light on an open question whether galaxy mass assembly has advanced in such regions. We found that a HzRG at z=4.72, HSC J0839+0113, spatially associates with an 4.7σ overdense region of Lyman break galaxies (LBGs) at z∼5 (r-dropouts) by using the HSC-SSP data. The system is found to be the most overdense region of LBGs among the known overdense regions with LBGs at the same epoch. No r-dropouts with blue color of i−z<∼0.3 are also found in the HSC J0839+0113 overdense region, while there are many such galaxies in the field. This result suggests that the galaxies may be dusty and have advanced stellar mass assembly in the overdense region. In this proposal, we will take a Keck/DEIMOS MOS observation for the surrounding r-dropouts around HSC J0839+0113. By this observation, we obtain the exact redshifts of the r-dropouts to identify the existence of the overdense region. Especially, we can evaluate the star formation rates (SFRs) of the target galaxies. Comparing the SFRs of the member galaxies of the overdense region with those of field galaxies allows us to test whether stellar mass assembly has advanced in the HSC J0839+0113 overdense region.
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