A Scaling Relation for the Giant Molecular Cloud Lifetime

The giant molecular cloud lifetime places an upper bound on the local time-scale for star-formation, linking cloud- and sub cloud-scale physics to galaxy-scale trends in the star formation rate. Here I present hydrodynamical simulations of Milky Way-like disc galaxies across galactic-dynamical environments in the moving-mesh code Arepo, containing a statistical sample of ~80,000 molecular clouds at a spatial resolution of 6 pc. Each simulation includes detailed sub-cloud physics (SN feedback, HII-region feedback, and ISM chemistry). I produce detailed cloud evolution networks for the entire cloud population of each galaxy and extract the characteristic molecular cloud lifetime, finding that it varies between 13 and 20 Myr across a range of spatial resolutions from 6 to 500 pc, in line with observations. The resulting ‘scaling relation’ for the cloud lifetime tells us that to resolve the cycle of cloud collapse, star formation and unbinding by stellar feedback, observations must probe down to scales smaller than the scale-height of the galactic gas disc (< 100 pc). This is now possible across a range of nearby galaxies. Contiguous molecular regions observed at resolutions larger than the gas-disc scale-height are neither bound nor collapsing, and are simply shaken apart on time-scales equal to the gas-disc turbulent crossing time.