Environmental impacts on molecular gas in protocluster galaxies at z ∼ 2

We present the results from ALMA CO(3–2) observations of 66 Hα-selected galaxies in three protoclusters around radio galaxies: PKS 1138−262 ($z$ = 2.16), USS 1558−003 ($z$ = 2.53), and 4C 23.56 ($z$ = 2.49). The pointing areas have an overdensity of ∼100 compared to the mean surface number density of galaxies in field environments. We detect the CO emission line in 16 star-forming galaxies, including six previously published galaxies, to measure the molecular gas mass. In the stellar mass range of 10.5 < log (Mstar/M⊙) < 11.0, the protocluster galaxies have larger gas mass fractions and longer gas depletion timescales compared to the scaling relations established for field galaxies. On the other hand, the amounts of molecular gas in more massive galaxies with log (Mstar/M⊙) > 11.0 are comparable in mass to the scaling relation, or smaller. Our results suggest that the environmental effects on gas properties are mass dependent: in high-density environments, gas accretion through cosmic filaments is accelerated in less massive galaxies, while this is suppressed in the most massive system.