  In a protocluster USS1558-003 at z=2.53, galaxies in the dense cores show
systematically elevated star-forming activities than those in less dense
regions. To understand its origin, we look into the gas properties of the
galaxies in the dense cores by conducting deep 1.1mm observations with Atacama
Large Millimeter/submillimeter Array (ALMA). We detect interstellar dust
continuum emissions from 12 member galaxies and estimate their molecular gas
masses. Comparing these gas masses with our previous measurements from CO(3-2)
line, we infer that the latter might be overestimated. We find that the gas to
stellar mass ratios of the galaxies in the dense cores tend to be higher (at M*
~ 10^{10} M_sun) where we see the enhanced star-forming activities), suggesting
that such large gas masses can sustain their high star-forming activities.
However, if we compare the gas properties of these protocluster galaxies with
the gas scaling relations constructed for field galaxies at similar cosmic
epoch, we find no significant environmental difference at the same stellar mass
and star formation rate. Although both gas mass ratios and star-forming
activities are enhanced in the majority of member galaxies, they appear to
follow the same scaling relation as field galaxies. Our results are consistent
with the scenario that the cold gas is efficiently supplied to protocluster
cores and to galaxies therein along surrounding filamentary structures, which
leads to the high gas mass fractions and thus the elevated star-formation
activities, but without changing the star formation law.
