Combined CO and Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star-formation Rate, and Stellar Mass

We combine molecular gas masses inferred from CO emission in 500 star-forming galaxies (SFGs) between z = 0 and 3, from the IRAM-COLDGASS, PHIBSS1/2, and other surveys, with gas masses derived from Herschel far-IR dust measurements in 512 galaxy stacks over the same stellar mass/redshift range. We constrain the scaling relations of molecular gas depletion timescale (t(depl)) and gas to stellar mass ratio (M-mol (gas)/M*) of SFGs near the star formation "main-sequence" with redshift, specific star-formation rate (sSFR), and stellar mass (M*). The CO- and dust-based scaling relations agree remarkably well. This suggests that the CO -> H-2 mass conversion factor varies little within +/- 0.6 dex of the main sequence (sSFR(ms, z, M*)), and less than 0.3 dex throughout this redshift range. This study builds on and strengthens the results of earlier work. We find that t(depl) scales as (1 + z)(-0.3) x (sSFR/sSFR(ms, z, M*))(-0.5), with little dependence on M*. The resulting steep redshift dependence of M-mol (gas)/M-* approximate to (1 + z)(3) mirrors that of the sSFR and probably reflects the gas supply rate. The decreasing gas fractions at high M-* are driven by the flattening of the SFR-M-* relation. Throughout the probed redshift range a combination of an increasing gas fraction and a decreasing depletion timescale causes a larger sSFR at constant M-*. As a result, galaxy integrated samples of the M-mol gas-SFR rate relation exhibit a super-linear slope, which increases with the range of sSFR. With these new relations it is now possible to determine M-mol gas with an accuracy of +/- 0.1 dex in relative terms, and +/- 0.2 dex including systematic uncertainties.