On the Gas Content, Star Formation Efficiency, and Environmental
  Quenching of Massive Galaxies in Proto-Clusters at z~2.0-2.5
  We present ALMA Band 6 (nu=233GHz, lambda=1.3mm) continuum observations
towards 68 'normal' star-forming galaxies within two Coma-like progenitor
structures at z=2.10 and 2.47, from which ISM masses are derived, providing the
largest census of molecular gas mass in overdense environments at these
redshifts. Our sample comprises galaxies with a stellar mass range of
1x10^9M_sun - 4x10^11M_sun with a mean M_*~6x10^10M_sun. Combining these
measurements with multiwavelength observations and SED modeling, we
characterize the gas mass fraction and the star formation efficiency, and infer
the impact of the environment on galaxies' evolution. Most of our detected
galaxies (~70%) have star formation efficiencies and gas fractions similar to
those found for coeval field galaxies and in agreement with the field scaling
relations. However, we do find that the proto-clusters contain an increased
fraction of massive, gas-poor galaxies, with low gas fractions (f_gas<6-10%)
and red rest-frame ultraviolet/optical colors typical of post-starburst and
passive galaxies. The relatively high abundance of passive galaxies suggests an
accelerated evolution of massive galaxies in proto-cluster environments. The
large fraction of quenched galaxies in these overdense structures also implies
that environmental quenching takes place during the early phases of cluster
assembly, even before virialization. From our data, we derive a quenching
efficiency of E_q~0.45 and an upper limit on the quenching timescale of
T_q<1Gyr.
