We present ALMA Band 6 ( \nu = 233 GHz , \lambda = 1.3 mm ) 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 1 \times 10 ^ { 9 } M _ { \odot } -4 \times 10 ^ { 11 } M _ { \odot } with a mean M _ { \star } \approx 6 \times 10 ^ { 10 } M _ { \odot } .
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 ( \gtrsim 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 } \lesssim 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 \epsilon _ { q } \approx 0.45 and an upper limit on the quenching timescale of \tau _ { q } < 1 Gyr .