Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks

TL;DR

This study uses high-resolution 2D-MHD simulations to show that infalling clumpy gas can sustain transonic turbulence in the outer regions of galactic disks, matching observed HI line widths.

Contribution

It demonstrates that gas accretion from high velocity clouds can maintain turbulence in galactic outskirts, providing a physical mechanism supported by numerical simulations.

Findings

Infall of high velocity clouds sustains transonic turbulence in HI gas.

HI line width is approximately 6.5 km/s at 2500 K due to accretion.

Radial variation of velocity dispersion discussed.

Abstract

High--resolution 2D--MHD numerical simulations have been carried out to investigate the effects of continuing infall of clumpy gas in extended HI galactic disks. Given a certain accretion rate, the response of the disk depends on its surface gas density and temperature. For Galactic conditions at a galactocentric distance of ~20 kpc, and for mass accretion rates consistent with current empirical and theoretical determinations in the Milky Way, the rain of compact high velocity clouds onto the disk can maintain transonic turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only mechanism of driving turbulence. Some statistical properties of the resulting forcing flow are shown in this Letter. The radial dependence of the gas velocity dispersion is also discussed.