Enhanced Star Formation of Less Massive Galaxies in a Proto-Cluster at
  z=2.5
  We investigate a correlation between star-formation rate (SFR) and stellar
mass for Halpha emission line galaxies (HAEs) in one of the richest
proto-clusters ever known at z~2.5, USS 1558-003 proto-cluster. This study is
based on a 9.7-hour narrow-band imaging data with MOIRCS on the Subaru
telescope. We are able to construct a sample, in combination with additional
H-band data taken with WFC3 on Hubble Space Telescope (HST), of 100 HAEs
reaching the dust-corrected SFRs down to 3 Msun/yr and the stellar masses down
to $10^{8.0}$ Msun. We find that while the star-forming galaxies with
>$10^{9.3}$ Msun are located on the universal SFR-mass main sequence
irrespective of the environment, less massive star-forming galaxies with
<$10^{9.3}$ Msun show a significant upward scatter from the main sequence in
this proto-cluster. This suggests that some less massive galaxies are in a
starburst phase, although we do not know yet if this is due to environmental
effects.
