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Validation of the equilibrium model for galaxy evolution to z~3 through molecular gas and dust observations of lensed star-forming galaxies

We combine IRAM Plateau de Bure Interferometer and Herschel PACS and SPIRE measurements to study the dust and gas contents of high-redshift star forming galaxies. We present new observations for a sample of 17 lensed galaxies at z=1.4-3.1, which allow us to directly probe the cold ISM of normal star-forming galaxies with stellar masses of ~10^10Msun, a regime otherwise not (yet) accessible by individual detections in Herschel and molecular gas studies. The lensed galaxies are combined with reference samples of sub-millimeter and normal z~1-2 star-forming galaxies with similar far-infrared photometry to study the gas and dust properties of galaxies in the SFR-M*-redshift parameter space. The mean gas depletion timescale of main sequence galaxies at z>2 is measured to be only ~450Myr, a factor of ~1.5 (~5) shorter than at z=1 (z=0), in agreement with a (1+z)^-1 scaling. The mean gas mass fraction at z=2.8 is 40+/-15% (44% after incompleteness correction), suggesting a flattening or even a reversal of the trend of increasing gas fractions with redshift recently observed up to z~2. The depletion timescale and gas fractions of the z>2 normal star-forming galaxies can be explained under the "equilibrium model" for galaxy evolution, in which the gas reservoir of galaxies is the primary driver of the redshift evolution of specific star formation rates. Due to their high star formation efficiencies and low metallicities, the z>2 lensed galaxies have warm dust despite being located on the star formation main sequence. At fixed metallicity, they also have a gas-to-dust ratio 1.7 times larger than observed locally when using the same standard techniques, suggesting that applying the local calibration of the relation between gas-to-dust ratio and metallicity to infer the molecular gas mass of high redshift galaxies may lead to systematic differences with CO-based estimates.