Molecular gas in intermediate-redshift ultraluminous infrared galaxies

We report on the results of observations in the CO(1-0) transition of a complete sample of Southern, intermediate-redshift (z = 0.2-0.5) ultraluminous infrared galaxies (ULIRGs) using the Mopra 22-m telescope. The 11 ULIRGs with L-FIR > 10(12.5) L-circle dot south of delta = -12 degrees were observed with integration times that varied between 5 and 24 h. Four marginal detections were obtained for individual targets in the sample. The 'stacked' spectrum of the entire sample yields a high significance, 10 sigma detection of the CO(1-0) transition at an average redshift of z = 0.38. The tightest correlation of L-FIR and L-CO for published low-redshift ULIRG samples (z < 0.2) is obtained after normalization of both these measures to a fixed dust temperature. With this normalization the relationship is linear. The distribution of dust-to-molecular hydrogen gas mass displays a systematic increase in dust-to-gas mass with galaxy luminosity for low-redshift samples, but this ratio declines dramatically for intermediate-redshift ULIRGs down to values comparable to that of the Small Magellanic Cloud. The upper envelope to the distribution of ULIRG molecular mass as function of look-back time demonstrates a dramatic rise by almost an order of magnitude from the current epoch out to 5 Gyr. This increase in maximum ULIRG gas mass with look-back time is even more rapid than that of the star formation rate density.