We propose a spectroscopic follow-up observation for candidates of ultra-faint dwarf (UFD) galaxies in the Milky Way, Cetus III and Pisces III, which have been newly discovered by the Subaru/Hyper Suprime-Cam (HSC). Cetus III and Pisces III are one of the faintest galaxies discovered so far. Multi-epoch observation for candidate member stars with medium-resolution spectroscopy leads to the identification of the genuine member stars, from which we can derive their dynamical mass (star + dark matter) and metallicity ([Fe/H]). Based on these results, we will clarify the star formation and chemical evolution history of the galaxies at the fainter end, which needs to be validated. Also, with this confirmation, these UFDs will provide strong observational constraints on the standard ΛCDM cosmology as highly dark matter-dominated systems. With future high-dispersion spectroscopy and astrometry (e.g., TMT, Roman), these UFDs would be ideal targets for testing not only galaxy formation on a small scale but also dark matter models.
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