We propose a MOIRCS NB2315 narrowband observation for a candidate cluster of galaxies at z=2.515, CC-0958, where a giant Lyα halo extending over ∼ 300 kpc has been discovered recently. The presence of the extended Lyα emission can be interpreted as evidence of the “cold-stream” mode of gas accretion, which is expected to sustain active star formation in CC-0958 undergoing the transition to hot gas accretion mode. The structure was first identified as a concentration of quiescent galaxies based on photometric redshifts, but no secure member identification has been made so far. By using the MOIRCS/NB2315 data combined with the rich ancillary data available in the COSMOS field, we will efficiently search for Hα emitting star-forming galaxies as robust members of CC-0958 down to a star formation rate of ∼ 5 M⊙ yr−1 to the extent of >∼ 1 Mpc. We will study the spatial distribution and degree of star formation activity of the Hα emitters depending on the environment from the cluster core to in and out of the possible filamentary structures around CC-0958. By comparing the results with other clusters/groups with Lyα halos, the proposed observation will provide a new perspective on the environmental effect in the context of modes of gas accretion.
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