Variation of galactic cold gas reservoirs with stellar mass

The stellar and neutral hydrogen (H I) mass functions at z ˜ 0 are fundamental benchmarks for current models of galaxy evolution. A natural extension of these benchmarks is the two-dimensional distribution of galaxies in the plane spanned by stellar and H I mass, which provides a more stringent test of simulations, as it requires the H I to be located in galaxies of the correct stellar mass. Combining H I data from the Arecibo Legacy Fast ALFA survey, with optical data from Sloan Digital Sky Survey, we find a distinct envelope in the H I-to-stellar mass distribution, corresponding to an upper limit in the H I fraction that varies monotonically over five orders of magnitude in stellar mass. This upper envelope in H I fraction does not favour the existence of a significant population of dark galaxies with large amounts of gas but no corresponding stellar population. The envelope shows a break at a stellar mass of ˜109 M⊙, which is not reproduced by modern models of galaxy populations tracing both stellar and gas masses. The discrepancy between observations and models suggests a mass dependence in gas storage and consumption missing in current galaxy evolution prescriptions. The break coincides with the transition from galaxies with predominantly irregular morphology at low masses to regular discs at high masses, as well as the transition from cold to hot accretion of gas in simulations.