A star-bursting proto-cluster in making associated to a radio galaxy at
  z=2.53 discovered by H_alpha imaging
  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 H_alpha imaging with MOIRCS on
Subaru Telescope. We find 68 H_alpha emitters with dust-uncorrected SFRs down
to 8.6 Msun/yr. Their spatial distribution indicates that there are three
prominent clumps of H_alpha 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 H_alpha 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 H_alpha 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
H_alpha 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 H_alpha 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.
