  In Meidt et al. (2018), we showed that gas kinematics on the scale of
individual molecular clouds are not dominated by self-gravity but also track a
component that originates with orbital motion in the potential of the host
galaxy. This agrees with observed cloud line widths, which show systematic
variations from virial motions with environment, pointing at the influence of
the galaxy potential. In this paper, we hypothesize that these motions act to
slow down the collapse of gas and so help regulate star formation. Extending
the results of Meidt et al. (2018), we derive a dynamical collapse timescale
that approaches the free-fall time only once the gas has fully decoupled from
the galactic potential. Using this timescale we make predictions for how the
fraction of free-falling, strongly self-gravitating gas varies throughout the
disks of star-forming galaxies. We also use this collapse timescale to predict
variations in the molecular gas star formation efficiency, which is lowered
from a maximum, feedback-regulated level in the presence of strong coupling to
the galactic potential. Our model implies that gas can only decouple from the
galaxy to collapse and efficiently form stars deep within clouds. We show that
this naturally explains the observed drop in star formation rate per unit gas
mass in the Milky Way's CMZ and other galaxy centers. The model for a galactic
bottleneck to star formation also agrees well with resolved observations of
dense gas and star formation in galaxy disks and the properties of local
clouds.
