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

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 $t_{\rm ff}\approx 1/ \sqrt{G \rho_{\rm disc}}$andthecoolingtimedeterminedbythecloudcollisionprocess$t_{\rm cool}$. This ratio is related to the ratio between the mean surface density of the disc ${\mathrm{\Sigma }}_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}}$ and the mean surface density of molecular clouds in the disc ${\mathrm{\Sigma }}_{\mathrm{c}\mathrm{l}\mathrm{o}\mathrm{u}\mathrm{d}}$. When $D<1\mathrm{/}3$ (which roughly corresponds to ${\mathrm{\Sigma }}_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}}<1\mathrm{/}3{\mathrm{\Sigma }}_{\mathrm{c}\mathrm{l}\mathrm{o}\mathrm{u}\mathrm{d}}$), cloud collision cooling is inefficient, and fragmentation is suppressed. When $D>1\mathrm{/}3$ (which roughly corresponds to ${\mathrm{\Sigma }}_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}}>1\mathrm{/}3{\mathrm{\Sigma }}_{\mathrm{c}\mathrm{l}\mathrm{o}\mathrm{u}\mathrm{d}}$), 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 subgrid model to simulate the global evolution of disc galaxies.