2011ApJ...730L..19G

We present IRAM Plateau de Bure interferometric detections of CO (J = 1 ⟶ 0) emission from a 24 µm-selected sample of star-forming galaxies at z = 0.4. The galaxies have polycyclic aromatic hydrocarbon 7.7 µm-derived star formation rates of SFR ∼30-60 M_☉/yr and stellar masses M_{sstarf}∼ 10¹¹ M_☉. The CO (J = 1 ⟶ 0) luminosities of the galaxies imply that the disks still contain a large reservoir of molecular gas, contributing ∼20% of the baryonic mass, but have star formation "efficiencies" similar to local quiescent disks and gas-dominated disks at z ∼ 1.5-2. We reveal evidence that the average molecular gas fraction has undergone strong evolution since z ∼ 2, with f_gas∝ (1 + z)^∼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 a ΛCDM universe are supported by these new observations.