A³COSMOS: Dissecting the gas content of star-forming galaxies across the main sequence at 1.2 z < 1.6

Aims. We aim to understand the physical mechanisms that drive star formation in a sample of mass-complete (>10(9.5) M-circle dot) star-forming galaxies (SFGs) at 1.2 <= z < 1.6.Methods. We selected SFGs from the COSMOS2020 catalog and applied a u upsilon-domain stacking analysis to their archival Atacama Large Millimeter/submillimeter Array (ALMA) data. Our stacking analysis provides precise measurements of the mean molecular gas mass and size of SFGs down to a stellar mass of M-star similar to 10(9.5) M-circle dot, even though at these stellar mass galaxies on the main sequence (MS) are no longer detected individually in the archival ALMA data. We also applied an image-domain stacking analysis on their HST i-band and UltraVISTA J - and K-s-band images. This allowed us to trace the distribution of their stellar component. Correcting these rest-frame optical sizes using the Rhalf-stellar-light-to-Rhalf-stellar-mass conversion at rest 5000 angstrom, we obtain the stellar mass size of MS galaxies and compare them to the sizes of their star-forming component obtained from our ALMA stacking analysis. Results. Across the MS (-0.2 < triangle MS = log(SFR/SFRMS) < 0.2), the mean molecular gas fraction of SFGs increases by a factor of similar to 1.4, while their mean molecular gas depletion time decreases by a factor of similar to 1.8. The scatter of the MS could thus be caused by variations in both the star formation efficiency and molecular gas fraction of galaxies. The mean molecular gas fraction of MS galaxies decreases by a factor of similar to 7 from M-star similar to 10(9.7) M-circle dot to similar to 10(11.3) M-circle dot, while their mean molecular gas depletion time remains roughly the same at all stellar masses. This finding could be a hint that the bending of the MS at z similar to 1.4 is primarily driven by variations in cold gas accretion. The majority of the galaxies lying on the MS have R-FIR approximate to R-stellar. Their central regions are subject to large dust attenuation. Starbursts (SBs, triangle MS > 0.7) have a mean molecular gas fraction similar to 2.1 times larger and mean molecular gas depletion time similar to 3.3 times shorter than MS galaxies. Additionally, they have more compact star-forming regions (similar to 2.5 kpc for MS galaxies vs. similar to 1.4 kpc for SBs) and systematically disturbed rest-frame optical morphologies, which is consistent with their association with major-mergers. SBs and MS galaxies follow the same relation between their molecular gas mass and star formation rate surface densities with a slope of similar to 1.1-1.2, that is, the so-called Kennicutt-Schmidt relation.