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

## TL;DR

This paper proposes that molecular cloud collisions cause dynamical cooling in gas-rich galactic discs, facilitating gravitational instability and clump formation, and introduces a parameter to quantify this cooling process.

## Contribution

It introduces a new model linking molecular cloud collisions to disc cooling and instability, providing a quantitative dissipation parameter for galaxy simulations.

## Key findings

- Cloud collision cooling is effective when disc surface density exceeds one-third of cloud surface density.
- Dynamical cooling via cloud collisions can trigger clump formation in gas-rich discs.
- The model can be incorporated into numerical simulations as a subgrid process.

## 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 $t_{\rm ff}\approx 1/ \sqrt{G \rho_{\rm disc}}$ 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.