A general theory for the lifetimes of giant molecular clouds under the
  influence of galactic dynamics
  We propose a simple analytic theory for environmentally-dependent molecular
cloud lifetimes, based on the large-scale (galactic) dynamics of the
interstellar medium. Within this theory, the cloud lifetime is set by the
time-scales for gravitational collapse, galactic shear, spiral arm
interactions, epicyclic perturbations and cloud-cloud collisions. It is
dependent on five observable quantities, accessible through measurements of the
galactic rotation curve, the gas and stellar surface densities, and the gas and
stellar velocity dispersions of the host galaxy. We determine how the relative
importance of each dynamical mechanism varies throughout the space of
observable galactic properties, and conclude that gravitational collapse and
galactic shear play the greatest role in setting the cloud lifetime for the
considered range of galaxy properties, while cloud-cloud collisions exert a
much lesser influence. All five environmental mechanisms are nevertheless
required to obtain a complete picture of cloud evolution. We apply our theory
to the galaxies M31, M51, M83, and the Milky Way, and find a strong dependence
of the cloud lifetime upon galactocentric radius in each case, with a typical
cloud lifetime between 10 and 50 Myr. Our theory is ideally-suited for
systematic observational tests with the Atacama Large Millimetre/submillimetre
array.
