Dynamical cooling of galactic discs by molecular cloud collisions – origin of giant clumps in gas-rich galaxy discs

Abstract

Different from Milky Way-like galaxies, discs of gas-rich galaxies are clumpy. It is believed that the clumps form because of gravitational instability. However, a necessary condition for gravitational instability to develop is that the disc must dissipate its kinetic energy effectively, this energy dissipation (also called cooling) is not well understood. We propose that collisions (coagulation) between molecular clouds dissipate the kinetic energy of the discs, which leads to a dynamical cooling. The effectiveness of this dynamical cooling is quantified by the dissipation parameter D, which is the ratio between the free-fall time |$t_{\rm ff}\approx 1/ \sqrt{G \rho _{\rm {\rm disc}}}$| and the cooling time determined by the cloud collision process tcool. This ratio is related to the ratio between the mean surface density of the disc Σdisc and the mean surface density of molecular clouds in the disc Σcloud. When D < 1/3 (which roughly corresponds to |$\Sigma _{\rm {\rm disc}} < 1/3 \Sigma _{\rm cloud}$|⁠), cloud collision cooling is inefficient, and fragmentation is suppressed. When D > 1/3 (which roughly corresponds to Σdisc > 1/3Σcloud), cloud–cloud collisions lead to a rapid cooling through which clumps form. On smaller scales, cloud–cloud collisions can drive molecular cloud turbulence. This dynamical cooling process can be taken into account in numerical simulations as a sub-grid model to simulate the global evolution of disc galaxies.