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

摘要

We combine IRAM Plateau de Bure Interferometer and Herschel PACS and SPIREmeasurements to study the dust and gas contents of high-redshift star forminggalaxies. We present new observations for a sample of 17 lensed galaxies atz=1.4-3.1, which allow us to directly probe the cold ISM of normal star-forminggalaxies with stellar masses of ~10^10Msun, a regime otherwise not (yet)accessible by individual detections in Herschel and molecular gas studies. Thelensed galaxies are combined with reference samples of sub-millimeter andnormal z~1-2 star-forming galaxies with similar far-infrared photometry tostudy the gas and dust properties of galaxies in the SFR-M*-redshift parameterspace. The mean gas depletion timescale of main sequence galaxies at z>2 ismeasured 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 is40+/-15% (44% after incompleteness correction), suggesting a flattening or evena reversal of the trend of increasing gas fractions with redshift recentlyobserved up to z~2. The depletion timescale and gas fractions of the z>2 normalstar-forming galaxies can be explained under the "equilibrium model" for galaxyevolution, in which the gas reservoir of galaxies is the primary driver of theredshift evolution of specific star formation rates. Due to their high starformation efficiencies and low metallicities, the z>2 lensed galaxies have warmdust despite being located on the star formation main sequence. At fixedmetallicity, they also have a gas-to-dust ratio 1.7 times larger than observedlocally when using the same standard techniques, suggesting that applying thelocal calibration of the relation between gas-to-dust ratio and metallicity toinfer the molecular gas mass of high redshift galaxies may lead to systematicdifferences with CO-based estimates.