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

Abstract: 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 $\sim$300$\mu$m continuum observed at 240 GHz for 45 massive ($\rm \langle log(M_{\star}(M_{\odot}))\rangle=10.7$), normal star forming ($\rm \langle log(sSFR(yr^{-1}))\rangle=-8.6$), i.e. MS, galaxies at $\rm z\approx3.2$ in the COSMOS field. From an empirical calibration between cold neutral, i.e. molecular and atomic, gas mass $\rm M_{gas}$ and monochromatic (rest-frame) infrared luminosity, the gas mass for this sample is derived. Combined with stellar mass $\rm M_{\star}$ and star formation rate (SFR) estimates (from {\sc MagPhys} fits) we obtain a median gas fraction of $\rm \mu_{gas}=M_{gas}/M_{\star}=1.65_{-0.19}^{+0.18}$ and a median gas depletion time $\rm t_{depl.}(Gyr)=M_{gas}/SFR=0.68_{-0.08}^{+0.07}$; correction for the location on the MS will only slightly change the values. The reported uncertainties are the $\rm 1\sigma$ 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 that 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 main sequence galaxies. While $\rm t_{dep.}$ shows only a mild dependence on location within the MS, a clear trend of increasing $\rm \mu_{gas}$ across the MS is observed (as known from previous studies). Further we comment on trends within the MS and (in)consistencies with other studies.