Global dynamics of the interstellar medium in magnetised disc galaxies

TL;DR

This study uses global magnetohydrodynamical simulations to explore how magnetic fields influence gas dynamics, fragmentation, and star formation in disc galaxies, revealing that magnetisation significantly alters gas structures and cloud formation.

Contribution

It provides new insights into the role of magnetic fields in galaxy evolution by demonstrating their impact on gas fragmentation and molecular cloud formation without stellar feedback.

Findings

Magnetic fields cause earlier fragmentation due to Parker instability.

Cold, dense gas forms above/below the midplane without feedback.

Magnetised discs develop filamentary structures that rapidly form giant molecular clouds.

Abstract

Magnetic fields are an elemental part of the interstellar medium in galaxies. However, their impact on gas dynamics and star formation in galaxies remains controversial. We use a suite of global magnetohydrodynamical simulations of isolated disc galaxies to study the influence of magnetic fields on the diffuse and dense gas in the discs. We find that the magnetic field acts in multiple ways. Stronger magnetised discs fragment earlier due to the shorter growth time of the Parker instability. Due to the Parker instability in the magnetised discs we also find cold ($T<50\,\mathrm{K}$) and dense ($n\sim10^3-10^4\,\mathrm{cm}^{-3}$) gas several hundred pc above/below the midplane without any form of stellar feedback. In addition, magnetic fields change the fragmentation pattern. While in the hydrodynamical case, the disc breaks up into ring-like structures, magnetised discs show the formation of filamentary entities that extent both in the azimuthal and radial direction. These kpc scale filaments become magnetically (super-)critical very quickly and allow for the rapid formation of massive giant molecular clouds. Our simulations suggest that major differences in the behaviour of star formation - due to a varying magnetisation - in galaxies could arise.