We investigate a correlation between star formation rate (SFR) and stellar mass for Hα emission-line galaxies (HAEs) in one of the richest protoclusters ever known at z ∼ 2.5, the USS 1558-003 protocluster. This study is based on a 9.7 hr narrowband 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 the Hubble Space Telescope, of 100 HAEs reaching the dust-corrected SFRs down to 3 M o yr-1 and the stellar masses down to 108.0 M o. We find that while the star-forming galaxies with 109.3 M o are located on the universal SFR-mass main sequence (MS) irrespective of the environment, less massive star-forming galaxies with ≲109.3 M o show a significant upward scatter from the MS in this protocluster. 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.
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