Large scale galactic turbulence: can self-gravity drive the observed HI velocity dispersions?

TL;DR

This study uses high-resolution simulations to investigate whether gravitational and hydrodynamic processes alone can explain the observed velocity dispersions in galactic HI gas, highlighting the role of self-gravity and feedback.

Contribution

It demonstrates that gravitational and hydrodynamic instabilities can account for HI turbulence without requiring supernova feedback at low star formation rates.

Findings

Self-gravity drives turbulence consistent with observed velocity dispersions.

Supernova feedback becomes significant at higher star formation rates.

Turbulence arises from large-scale modes and cloud interactions.

Abstract

Observations of turbulent velocity dispersions in the HI component of galactic disks show a characteristic floor in galaxies with low star formation rates and within individual galaxies the dispersion profiles decline with radius. We carry out several high resolution adaptive mesh simulations of gaseous disks embedded within dark matter haloes to explore the roles of cooling, star-formation, feedback, shearing motions and baryon fraction in driving turbulent motions. In all simulations the disk slowly cools until gravitational and thermal instabilities give rise to a multi-phase medium in which a large population of dense self-gravitating cold clouds are embedded within a warm gaseous phase that forms through shock heating. The diffuse gas is highly turbulent and is an outcome of large scale driving of global non-axisymmetric modes as well as cloud-cloud tidal interactions and merging. At low star-formation rates these processes alone can explain the observed HI velocity dispersion profiles and the characteristic value of ~10 km/s observed within a wide range of disk galaxies. Supernovae feedback creates a significant hot gaseous phase and is an important driver of turbulence in galaxies with a star-formation rate per unit area >10^-3 M_sun/yr/kpc^2.