# On the effective turbulence driving mode of molecular clouds formed in disc galaxies

## TL;DR

This study uses high-resolution simulations to analyze turbulence driving modes in molecular clouds within disc galaxies, revealing a broad range of driving modes and the importance of resolution for accurate measurements.

## Contribution

It introduces a detailed analysis of turbulence driving modes in molecular clouds formed in galaxy simulations, highlighting the variability and resolution dependence of the driving parameter.

## Key findings

- The turbulence driving parameter b varies from solenoidal to compressive modes.
- b values are lower limits and increase with simulation resolution.
- The effective turbulence driving mode differs significantly among clouds.

## Abstract

We determine the physical properties and turbulence driving mode of molecular clouds formed in numerical simulations of a Milky Way-type disc galaxy with parsec-scale resolution. The clouds form through gravitational fragmentation of the gas, leading to average values for mass, radii and velocity dispersion in good agreement with observations of Milky Way clouds. The driving parameter (b) for the turbulence within each cloud is characterised by the ratio of the density contrast (sigma_rho) to the average Mach number (Mach) within the cloud, b = sigma_rho/Mach. As shown in previous works, b ~ 1/3 indicates solenoidal (divergence-free) driving and b ~ 1 indicates compressive (curl-free) driving. We find that the average b value of all the clouds formed in the simulations has a lower limit of b > 0.2. Importantly, we find that b has a broad distribution, covering values from purely solenoidal to purely compressive driving. Tracking the evolution of individual clouds reveals that the b value for each cloud does not vary significantly over their lifetime. Finally, we perform a resolution study with minimum cell sizes of 8, 4, 2 and 1 pc and find that the average b value increases with increasing resolution. Therefore, we conclude that our measured b values are strictly lower limits and that a resolution better than 1 pc is required for convergence. However, regardless of the resolution, we find that b varies by factors of a few in all cases, which means that the effective driving mode alters significantly from cloud to cloud.