Galaxy evolution and star formation efficiency at 0.2 < z < 0.6

We present the results of a CO line survey of 30 galaxies at moderate redshift (z similar to 0.2-0.6), with the IRAM 30 m telescope, with the goal to follow galaxy evolution and in particular the star formation efficiency (SFE) as defined by the ratio between far-infrared luminosity and molecular gas mass (L-FIR/M(H-2)). The sources are selected to be ultra-luminous infra-red galaxies (ULIRGs), with L-FIR larger than 2.8 x 10(12) L-circle dot, experiencing starbursts; adopting a low ULIRG CO-to-H-2 conversion factor, their gas consumption time-scale is lower than 10(8) yr. To date only very few CO observations exist in this redshift range that spans nearly 25% of the universe's age. Considerable evolution of the star formation rate is already observed during this period. 18 galaxies out of our sample of 30 are detected (of which 16 are new detections), corresponding to a detection rate of 60%. The average CO luminosity for the 18 galaxies detected is L-CO' = 2 x 10(10) K km s(-1) pc(2), corresponding to an average H-2 mass of 1.6 x 10(10) M-circle dot. The FIR luminosity correlates well with the CO luminosity, in agreement with the correlation found for low and high redshift ULIRGs. Although the conversion factor between CO luminosity and H-2 mass is uncertain, we find that the maximum amount of gas available for a single galaxy is quickly increasing as a function of redshift. Using the same conversion factor, the SFEs for z similar to 0.2-0.6 ULIRGs are found to be significantly higher, by a factor 3, than for local ULIRGs, and are comparable to high redshift ones. We compare this evolution to the expected cosmic H-2 abundance and the cosmic star formation history.