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We report the discovery of a significant excess of candidate Halpha emitters(HAEs) in the field of the radio galaxy 4C 23.56 at z=2.483. Using the MOIRCSnear-infrared imager on the Subaru Telescope we found 11 candidateemission-line galaxies to a flux limit of ~7.5 10^-17 erg s-1 cm-2, which isabout 5 times excess from the expected field counts with ~3-sigma significance.Three of these are spectroscopically confirmed as redshifted Halpha at z=2.49.The distribution of candidate emitters on the sky is tightly confined to a1.2-Mpc-radius area at z=2.49, locating 4C 23.56 at the western edge of thedistribution. Analysis of the deep Spitzer MIPS 24 mu m imaging shows thatthere is also an excess of faint MIPS sources. All but two of the 11 HAEs arealso found in the MIPS data. The inferred star-formation rate (SFR) of the HAEsbased on the extinction-corrected Halpha luminosity (median SFR >~100 M_solaryr-1) is similar to those of HAEs in random fields at z~2. On the other hand,the MIPS-based SFR for the HAEs is on average 3.6 times larger, suggesting theexistence of the star-formation significanly obscured by dust. The comparisonof the Halpha-based star-formation activities of the HAEs in the 4C 23.56 fieldto those in another proto-cluster around PKS 1138-262 at z=2.16 reveals thatthe latter tend to have fainter Halpha emission despite similar K-bandmagnitudes. This suggests that star-formation may be suppressed in the PKS1138-262 protocluster relative to the 4C 23.56 protocluster. This differenceamong the HAEs in the two proto-clusters at z > 2 may imply that some massivecluster galaxies are just forming at these epochs with some variation amongclusters.

Discovery of an Excess of Halpha Emitters around 4C 23.56 at z=2.48