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We report a discovery of a proto-cluster in vigorous assembly and hosting strong star forming activities, associated to a radio galaxy USS 1558-003 at z=2. 53, as traced by a wide-field narrow-band Hₐlpha imaging with MOIRCS on Subaru Telescope. We find 68 Hₐlpha emitters with dust-uncorrected SFRs down to 8. 6 Msun/yr. Their spatial distribution indicates that there are three prominent clumps of Hₐlpha emitters, one surrounding the radio galaxy and another located at ~1. 5 Mpc away to the south-west, and the other located in 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 later times. Whilst most Hₐlpha 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 Hₐlpha emitters are located towards the faint end of the red sequence, and they tend to be located in the high density clumps. We do not see any statistically significant difference in the distributions of individual star formation rates or stellar masses of the Hₐlpha 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 Hₐlpha emission of the radio galaxy is fairly extended spatially over ~4. 5 arcsec. However it is not as widespread as its Lya halo, meaning that the Lya emission is indeed severely extended by resonant scattering.
Hayashi et al. (Thu,) studied this question.