The turbulence driving parameter of molecular clouds in disc galaxies

TL;DR

This study uses magnetohydrodynamical simulations to analyze the evolution of turbulence driving modes in molecular clouds within disc galaxies, revealing mostly compressive driving with rapid mode changes influenced by cloud interactions.

Contribution

It provides new insights into the time evolution and environmental dependence of turbulence driving modes in molecular clouds through detailed simulations.

Findings

Ensemble-averaged turbulence driving parameter is compressive, around 0.5-0.7.

Individual clouds exhibit highly fluctuating driving modes.

Magnetic fields reduce fluctuations in the turbulence driving parameter.

Abstract

Supersonic turbulence plays a pivotal role during the formation of molecular clouds and stars in galaxies. However, little is known about how the fraction of compressive and solenoidal modes in the velocity field evolves over time and how it depends on properties of the molecular cloud or the galactic environment. In this work, we carry out magnetohydrodynamical simulations of disc galaxies and study the time evolution of the turbulence driving parameter for an ensemble of clouds. We find that the time-averaged turbulence driving parameter is insensitive to the position of the cloud within the galaxy. The ensemble-averaged driving parameter is found to be rather compressive with $b\sim0.5-0.7$, indicating almost time-independent global star formation properties. However, each individual cloud shows a highly fluctuating driving parameter, which would strongly affect the cloud's star formation rate. We find that the mode of turbulence driving can rapidly change within only a few Myr, both from solenoidal to compressive and vice versa. We attribute these changes to cloud collisions and to tidal interactions with clouds or overdensities in the environment. Last, we find no significant differences in the average driving parameter between hydrodynamic and initially strongly magnetised galaxies. However, the magnetic field tends to reduce the overall fluctuation of the driving parameter. The average driving as well as its uncertainty are seen to be in agreement with recent constraints on the turbulence driving mode for solar neighbourhood clouds.