Gas Fraction and Depletion Time of Massive Star-forming Galaxies at z ~ 3.2: No Change in Global Star Formation Process out to z > 3

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}_\star ({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}_\star and star formation rate (SFR) estimates (from MagPhys fits) we obtain a median gas fraction of {μ }_{gas}={M}_{gas}/{M}_\star ={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 1σ error 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.