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

R. Genzel, L. J. Tacconi, D. Lutz, A. Saintonge, S. Berta, B. Magnelli, F. Combes, S. García-Burillo, R. Neri, A. Bolatto, T. Contini, S. Lilly, J. Boissier, F. Boone, N. Bouché, F. Bournaud, A. Burkert, M. Carollo, L. Colina, M. C. Cooper, P. Cox, C. Feruglio, N. M. Förster Schreiber, J. Freundlich, J. Gracia-Carpio, S. Juneau, K. Kovac, M. Lippa, T. Naab, P. Salome, A. Renzini, A. Sternberg, F. Walter, B. Weiner, A. Weiss, S. Wuyts

Published 2014-09-03Version 1

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 time scale (tdepl) and gas fraction (Mmolgas/M*) with redshift, specific star formation rate (sSFR) and stellar mass (M*) in SFGs. The CO- and dust-based scaling relations agree remarkably well. This suggests that the CO-H2 mass conversion factor varies little within +-0.6 dex of the main sequence line, and less than a factor of 2 throughout this redshift range. We find that tdepl scales as (1+z)^-0.3 *(sSFR)^-0.5, with no M* dependence. The resulting steep redshift dependence of Mmolgas/M* ~ (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. At constant M*, a larger sSFR is due to a combination of an increasing gas fraction and a decreasing depletion time scale. As a result galaxy integrated samples of the Mmolgas-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 Mmolgas with an accuracy of +-0.1 dex in relative terms, and +-0.2 dex including systematic uncertainties.