  Using a series of high-resolution hydrodynamical simulations we show that
during the rapid growth of high-redshift (z > 5) galaxies, reserves of
molecular gas are consumed over a time-scale of 300Myr, almost independent of
feedback scheme. We find that there exists no such simple relation for the
total gas fractions of these galaxies, with little correlation between gas
fractions and specific star formation rates. The bottleneck or limiting factor
in the growth of early galaxies is in converting infalling gas to cold
star-forming gas. Thus, we find that the majority of high redshift dwarf
galaxies are effectively in recession, with demand (of star formation) never
rising to meet supply (of gas), irrespective of the baryonic feedback physics
modelled. We conclude that the basic assumption of self-regulation in galaxies
- that they can adjust total gas consumption within a Hubble time - does not
apply for the dwarf galaxies thought to be responsible for providing most UV
photons to reionize the high redshift Universe. We demonstrate how this rapid
molecular time-scale improves agreement between semi-analytic model predictions
of the early Universe and observed stellar mass functions.
