Gas Fraction and Depletion Time of Massive Star Forming Galaxies at $z \sim$ 3.2: No Change in Global Star Formation Process out to $z>$ 3
Résumé
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 ($\langle \mathrm{log}({M}_{\star }({M}_{\odot }))\rangle =10.7$), normal star-forming ($\langle \mathrm{log}(\mathrm{sSFR}({\mathrm{yr}}^{-1}))\rangle =-8.6$), i.e., MS, galaxies at $z\approx 3.2$ in the COSMOS field. From an empirical calibration between cold neutral, i.e., molecular and atomic, gas mass ${M}_{\mathrm{gas}}$ 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 ${\mu }_{\mathrm{gas}}={M}_{\mathrm{gas}}/{M}_{\star }={1.65}_{-0.19}^{+0.18}$ and a median gas depletion time ${t}_{\mathrm{depl}.}(\mathrm{Gyr})={M}_{\mathrm{gas}/\mathrm{SFR}}={0.68}_{-0.08}^{+0.07},$ correction for the location on the MS will only slightly change the values. The reported uncertainties are the $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 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 ${t}_{\mathrm{depl}.}$ shows only a mild dependence on location within the MS, a clear trend of increasing ${\mu }_{\mathrm{gas}}$ across the MS is observed (as known from previous studies). Further, we comment on trends within the MS and (in)consistencies with other studies.