How is cold, star-forming gas in galaxies affected by magnetic fields?

TL;DR

This study uses novel galaxy simulations to explore how magnetic fields influence the structure and star formation activity in cold, star-forming gas, revealing that magnetic support alters the gas distribution and star formation thresholds.

Contribution

First non-KS law galaxy simulations with magnetic fields, showing magnetic effects on gas morphology and star formation without imposing a predefined star formation relation.

Findings

Magnetic fields produce a more compact, magnetized galactic disc.

Magnetic support raises the gas surface density threshold for star formation.

Magnetic fields alter the gas distribution and star formation rate compared to non-magnetic models.

Abstract

Numerical simulations provide a unique opportunity to improve our understanding of the role of magnetic fields in the interstellar medium of galaxies and in star formation. However, many existing galaxy-scale numerical simulations impose a Kennicutt-Schmidt (KS) star formation law by construction. In this paper, we present two Arepo simulations of an isolated star-forming galaxy with and without magnetic fields, using sink particles to model star formation without imposing a KS relation. We examine global differences between the models, and investigate the impacts on star formation. We include a time-dependent, non-equilibrium chemical network coupled to a thermal evolution scheme and supernova feedback. Our magnetic field amplifies via dynamo action from a small initial seed field. We find a more compact magnetohydrodynamic (MHD) disc (radius ~ 5.1kpc, compared to ~ 7.4kpc), with a diffuse atomic envelope above and below the plane that is not seen in the hydrodynamic (HD) case. The HD disc displays a smoother, more even radial distribution of gas and star formation, and more bubbly substructure. Our MHD simulation has a higher proportion of dense, gravitationally unbound gas than the HD case, but a lower star formation rate, an average between 125-150Myr of ~ 4.8 solar masses per year, compared to ~ 8.4 solar masses per year. We see a clear shift in the KS relation to higher gas surface densities in the MHD case, more consistent with observations. The additional magnetic support against gravitational collapse seems to raise the threshold gas surface density required for star formation.