We intend to propose a spectroscopic follow-up observation for an ultra-faint dwarf (UFD) galaxy in the Milky Way, Boötes IV, which has been newly discovered by the Hyper Suprime-Cam (HSC) installed on the Subaru telescope. Boötes IV is one of the faintest galaxies reported so far, with an absolute magnitude of -5.36 in the V band and a distance from the Sun of 209 kpc. 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 its dynamical mass (star + dark matter) and metallicity ([Fe/H]). Based on these results, we will gain valuable insights into the star formation and chemical evolution history of the galaxy at the fainter end, which has not yet been thoroughly tested. Also, with this spectroscopic confirmation, Boötes IV will provide strong observational constraints in ΛCDM cosmology due to the high fraction of dark matter to baryon mass. With future high-dispersion spectroscopy and astrometry (e.g., TMT, Roman), this UFD would be an ideal target for testing dark matter models and galaxy formation on a small scale.
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