Fast Molecular Cloud Destruction Requires Fast Cloud Formation
  A large fraction of the gas in the Galaxy is cold, dense, and molecular. If
all this gas collapsed under the influence of gravity and formed stars in a
local free-fall time, the star formation rate in the Galaxy would exceed that
observed by more than an order of magnitude. Other star-forming galaxies behave
similarly. Yet observations and simulations both suggest that the molecular gas
is indeed gravitationally collapsing, albeit hierarchically. Prompt stellar
feedback offers a potential solution to the low observed star formation rate if
it quickly disrupts star-forming clouds during gravitational collapse. However,
this requires that molecular clouds must be short-lived objects, raising the
question of how so much gas can be observed in the molecular phase. This can
occur only if molecular clouds form as quickly as they are destroyed,
maintaining a global equilibrium fraction of dense gas. We therefore examine
cloud formation timescales. We first demonstrate that supernova and superbubble
sweeping cannot produce dense gas at the rate required to match the cloud
destruction rate. On the other hand, Toomre gravitational instability can reach
the required production rate. We thus argue that, although dense, star-forming
gas may last only around a single global free-fall time, the dense gas in
star-forming galaxies can globally exist in a state of dynamic equilibrium
between formation by gravitational instability, and disruption by stellar
feedback. At redshift z >~ 2, the Toomre instability timescale decreases,
resulting in a prediction of higher molecular gas fractions at early times, in
agreement with observations.
