The frequency and nature of `cloud-cloud collisions' in galaxies
  We investigate cloud-cloud collisions, and GMC evolution, in hydrodynamic
simulations of isolated galaxies. The simulations include heating and cooling
of the ISM, self--gravity and stellar feedback. Over timescales $<5$ Myr most
clouds undergo no change, and mergers and splits are found to be typically two
body processes, but evolution over longer timescales is more complex and
involves a greater fraction of intercloud material. We find that mergers, or
collisions, occur every 8-10 Myr (1/15th of an orbit) in a simulation with
spiral arms, and once every 28 Myr (1/5th of an orbit) with no imposed spiral
arms. Both figures are higher than expected from analytic estimates, as clouds
are not uniformly distributed in the galaxy. Thus clouds can be expected to
undergo between zero and a few collisions over their lifetime. We present
specific examples of cloud--cloud interactions in our results, including
synthetic CO maps. We would expect cloud--cloud interactions to be observable,
but find they appear to have little or no impact on the ISM. Due to a
combination of the clouds' typical geometries, and moderate velocity
dispersions, cloud--cloud interactions often better resemble a smaller cloud
nudging a larger cloud. Our findings are consistent with the view that spiral
arms make little difference to overall star formation rates in galaxies, and we
see no evidence that collisions likely produce massive clusters. However, to
confirm the outcome of such massive cloud collisions we ideally need higher
resolution simulations.
