2016ApJ...833..112S

The observed evolution of the gas fraction and its associated depletion time in main-sequence (MS) galaxies provides insights on how star formation proceeds over cosmic time. We report ALMA detections of the rest-frame ∼300 µm continuum observed at 240 GHz for 45 massive (<log(M_*(M_☉))>=10.7), normal star-forming (<log(sSFR(yr^–1))>=-8.6), i.e., MS, galaxies at z~3.2 in the COSMOS field. From an empirical calibration between cold neutral, i.e., molecular and atomic, gas mass M_gas and monochromatic (rest-frame) infrared luminosity, the gas mass for this sample is derived. Combined with stellar mass M_* and star formation rate (SFR) estimates (from MagPhys fits) we obtain a median gas fraction of µ_gas=M_gas/M_*=1.65_–0.19^+0.18 and a median gas depletion time t_depl.(Gyr)=M_gas/SFR=0.68_–0.08^+0.07; correction for the location on the MS will only slightly change the values. The reported uncertainties are the 1σerror on the median. Our results are fully consistent with the expected flattening of the redshift evolution from the 2-SFM (2 star formation mode) framework which empirically prescribes the evolution assuming a universal, log-linear relation between SFR and gas mass coupled to the redshift evolution of the specific star formation rate (sSFR) of MS galaxies. While t_depl. shows only a mild dependence on location within the MS, a clear trend of increasing µ_gas across the MS is observed (as known from previous studies). Further, we comment on trends within the MS and (in)consistencies with other studies.