The frequency and nature of `cloud-cloud collisions' in galaxies

TL;DR

This study uses hydrodynamic simulations to analyze the frequency, nature, and impact of cloud-cloud collisions in galaxies, revealing that such interactions are relatively infrequent and have limited influence on star formation and cluster formation.

Contribution

The paper provides the first detailed analysis of cloud-cloud collision rates and effects in simulated galaxies, incorporating realistic ISM physics and comparing spiral and non-spiral scenarios.

Findings

Cloud collisions occur every 8-10 Myr in spiral arms and every 28 Myr without arms.

Most clouds undergo no change over 5 Myr, with mergers being typically two-body processes.

Cloud interactions have minimal impact on the ISM and star formation rates.

Abstract

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.