We present IRAM Plateau de Bure interferometric detections of CO ( J = 1 \rightarrow 0 ) emission from a 24 \mu m-selected sample of star-forming galaxies at z = 0.4 .
The galaxies have PAH 7.7 \mu m-derived star formation rates of SFR \sim 30–60 M _ { \odot } yr ^ { -1 } and stellar masses M _ { \star } \sim 10 ^ { 11 } M _ { \odot } .
The CO ( J = 1 \rightarrow 0 ) luminosities of the galaxies imply that the disks still contain a large reservoir of molecular gas , contributing \sim 20 % of the baryonic mass , but have star-formation ‘ efficiencies ’ similar to local quiescent disks and gas-dominated disks at z \sim 1.5 –2 .
We reveal evidence that the average molecular gas fraction has undergone strong evolution since z \sim 2 , with f _ { gas } \propto ( 1 + z ) ^ { \sim 2 \pm 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 \Lambda CDM Universe are supported by these new observations .