Molecular Gas in Intermediate Redshift ULIRGs
  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)
Ultra-Luminous Infra-Red Galaxies using the Mopra 22m telescope. The eleven
ULIRGs with L_FIR > 10^12.5 L_Sun south of Dec = -12 deg were observed with
integration times that varied between 5 and 24 hours. 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 normalisation of both these measures to a fixed dust
temperature. With this normalisation 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.
