Wednesday

Three intimately linked observational tracers broadly characterise the cosmic evolution of galaxies: star-formation rate density, stellar mass density and molecular gas mass density. Our current understanding of the cosmic evolution of galaxies is dominated by comprehensive measurements of the former two, with a clear empirical picture emerging of an evolution of star-formation which rises rapidly to a peak around z~2 and then decays to the present day. Ultimately though it is the evolution of molecular gas which drives galaxy evolution as it fuels on-going star-formation and is the reservoir from which stars are assembled. In recent years direct measurements have been placed on the shape of the CO luminosity function which constrain the evolution of molecular gas mass density. However, observations of CO are time expensive and so measurements of molecular gas mass density from CO line surveys rely on small samples (1000) of galaxies, and hence statistical uncertainties on these measurements are large. We use a statistical approach to provide new empirical constraints on the evolution of the cosmological molecular gas mass density back to z~2.5, not by measuring the CO luminosity function directly, but by measuring average observed 850um flux density of a sample of ~150,000 near-infrared selected galaxies. The redshift range considered corresponds to a span where the 850um band probes the Rayleigh-Jeans tail of thermal dust emission in the rest-frame and can therefore be used as an estimate of the mass of the interstellar medium. With a sample ~2 orders of magnitude larger than in previous works we are able to significantly reduce statistical uncertainties on the molecular gas mas density to z~2.5. Our results suggest that evolution of the cold molecular gas content in galaxies mirrors the increase in cosmic star-formation rate towards its peak ~10 billion years ago and decline to the current day.