The physical origin of long gas depletion times in galaxies
  We present a model that explains why galaxies form stars on a time scale
significantly longer than the time scales of processes governing the evolution
of interstellar gas. We show that gas evolves from a non-star-forming to a
star-forming state on a relatively short time scale and thus the rate of this
evolution does not limit the star formation rate. Instead, the star formation
rate is limited because only a small fraction of star-forming gas is converted
into stars before star-forming regions are dispersed by feedback and dynamical
processes. Thus, gas cycles into and out of star-forming state multiple times,
which results in a long time scale on which galaxies convert gas into stars.
Our model does not rely on the assumption of equilibrium and can be used to
interpret trends of depletion times with the properties of observed galaxies
and the parameters of star formation and feedback recipes in simulations. In
particular, the model explains how feedback self-regulates the star formation
rate in simulations and makes it insensitive to the local star formation
efficiency. We illustrate our model using the results of an isolated
$L_*$-sized galaxy simulation that reproduces the observed Kennicutt-Schmidt
relation for both molecular and atomic gas. Interestingly, the relation for
molecular gas is almost linear on kiloparsec scales, although a nonlinear
relation is adopted in simulation cells. We discuss how a linear relation
emerges from non-self-similar scaling of the gas density PDF with the average
gas surface density.
