ON THE EVOLUTION OF THE MOLECULAR GAS FRACTION OF STAR-FORMING GALAXIES

We present IRAM Plateau de Bure interferometric detections of CO (J = 1 -> 0) emission from a 24 mu m-selected sample of star-forming galaxies at z = 0.4. The galaxies have polycyclic aromatic hydrocarbon 7.7 mu m-derived star formation rates of SFR similar to 30-60 M-circle dot yr(-1) and stellar masses M-star similar to 10(11) M-circle dot. The CO(J = 1 -> 0) luminosities of the galaxies imply that the disks still contain a large reservoir of molecular gas, contributing similar to 20% of the baryonic mass, but have star formation efficiencies similar to local quiescent disks and gas-dominated disks at z similar to 1.5-2. We reveal evidence that the average molecular gas fraction has undergone strong evolution since z similar to 2, with f(gas) alpha (1 + z)(similar to 2 +/- 0.5). The evolution of f(gas) encodes fundamental information about the relative depletion/replenishment of molecular fuel in galaxies and is expected to be a strong function of halo mass. We show that the latest predictions for the evolution of the molecular gas fraction in semi-analytic models of galaxy formation within Lambda CDM universe are supported by these new observations.