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 $t_{\rm ff}\approx 1/ \sqrt{G \rho_{\rm
disc}}$ and the cooling time determined by the cloud collision process $t_{\rm
cool}$. This ratio is related to the ratio between the mean surface density of
the disc $\Sigma_{\rm disc}$ and the mean surface density of molecular clouds
in the disc $\Sigma_{\rm cloud}$. When $D <1/3$ (which roughly corresponds to
$\Sigma_{\rm disc} < 1/3 \Sigma_{\rm cloud}$), cloud collision cooling is
inefficient, and fragmentation is suppressed. When $D > 1/3$ (which roughly
corresponds to $\Sigma_{\rm disc} > 1/3 \Sigma_{\rm cloud}$), 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.
