(Proposal ID) S11A-112 (PI) Tanaka, Ichi (Proposal Title) NIR Spectroscopy of Massive Forming Galaxies in the Core of Proto-cluster at z=2.5 (Abstract) We recently report a significant excess of candidate Hƒ¿ emitters (HAEs) in the field of the radio galaxy 4C 23.56 at z=2.49 (Tanaka et al., submitted). Using the custom-made narrowband filter which neatly captures the redshifted H alpha emission lines at z=2.49, we detected a clear excess of HAEs by a factor of 5 compared to general fields. The HAEs are tightly clustered within 1.2-Mpc- radius region adjacent to 4C 23.56. The analysis of the deep Spitzer MIPS 24 mu m imaging has also revealed a significant excess of the faint MIPS sources in the same area of the HAEs concentration. These results srongly indicate the existence of a protocluster associated to the radio galaxy, which contains dusty star-bursting massive galaxies. We here propose a deep near-infrared spectroscopy in order to confirm our discoveries by spectroscopic redshifts, and to look into the physical properties of member galaxies (such as extinction -corrected star-formation rate, metallicity, ionization source diagnostics, etc.). We will compare our results with those in the field environments at z > 2, in particular the HAEs in the GOODS-N that are selected by the same technique. We will be able to investigate directly the SF activities as a function of environment at the peak epoch of galaxy formation at z~2.5 for the first time.