  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 MOIRCS
near-infrared imager on the Subaru Telescope we found 11 candidate
emission-line galaxies to a flux limit of ~7.5 10^-17 erg s-1 cm-2, which is
about 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 a
1.2-Mpc-radius area at z=2.49, locating 4C 23.56 at the western edge of the
distribution. Analysis of the deep Spitzer MIPS 24 mu m imaging shows that
there is also an excess of faint MIPS sources. All but two of the 11 HAEs are
also found in the MIPS data. The inferred star-formation rate (SFR) of the HAEs
based on the extinction-corrected Halpha luminosity (median SFR >~100 M_solar
yr-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 the
existence of the star-formation significanly obscured by dust. The comparison
of the Halpha-based star-formation activities of the HAEs in the 4C 23.56 field
to those in another proto-cluster around PKS 1138-262 at z=2.16 reveals that
the latter tend to have fainter Halpha emission despite similar K-band
magnitudes. This suggests that star-formation may be suppressed in the PKS
1138-262 protocluster relative to the 4C 23.56 protocluster. This difference
among the HAEs in the two proto-clusters at z > 2 may imply that some massive
cluster galaxies are just forming at these epochs with some variation among
clusters.
