We report a discovery of a proto-cluster in vigorous assembly and hosting strong star-forming activities, associated with a radio galaxy USS 1558-003 at z = 2.53, as traced by wide-field narrow-band Hα imaging with MOIRCS on the Subaru Telescope. We find 68 Hα emitters with dust-uncorrected star formation rates (SFRs) down to 8.6 M ⊙yr-1. Their spatial distribution indicates that there are three prominent clumps of Hα emitters: one surrounding the radio galaxy, the second located at 1.5Mpc away to the southwest, and the third located between the two. These contiguous three systems are very likely to merge together in the near future and may grow to a single more massive cluster at a later time. While most Hα emitters reside in the "blue cloud" on the color-magnitude diagram, some emitters have very red colors with J - Ks > 1.38(AB). Interestingly, such red Hα emitters are located toward the faint end of the red sequence, and they tend to be located in high density clumps. We do not see any statistically significant difference in the distributions of individual SFRs or stellar masses of the Hα emitters between the dense clumps and the other regions, suggesting that this is one of the notable sites where the progenitors of massive galaxies in the present-day clusters were in their vigorous formation phase. Finally, we find that Hα emission of the radio galaxy is fairly extended spatially over 45. However, it is not as widespread as its Lyα halo, meaning that the Lyα emission is indeed severely extended by resonant scattering.
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