Variation of galactic cold gas reservoirs with stellar mass
  The stellar and neutral hydrogen (HI) 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 HI mass, which provides a more stringent test of simulations, as
it requires the HI to be located in galaxies of the correct stellar mass.
Combining HI data from the ALFALFA survey, with optical data from SDSS, we find
a distinct envelope in the HI-to-stellar mass distribution, corresponding to an
upper limit in the HI fraction that varies monotonically over five orders of
magnitude in stellar mass. This upper envelope in HI 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 ~10^9 Msun, 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 disks at high masses, as well as the
transition from cold to hot accretion of gas in simulations.
