The Effects of Magnetic Fields on the Star-Forming Interstellar Medium of Galaxies

Abstract

In this thesis, I present AREPO simulations of full isolated galaxy discs, with the aim of exploring the impact of magnetic fields on the star-forming Interstellar Medium (ISM). While it is known that stars form in the cold, dense gas of molecular clouds, how these clouds form and evolve is not fully understood. Determining the role of magnetic fields in this process has been a notoriously difficult challenge, but cutting-edge magnetohydrodynamic (MHD) galaxy simulations can help to answer this question. In Chapter 3, I present two primary galaxy models. The simulations are near-identical, bar the inclusion or exclusion of magnetic fields, yet I find significant differences between them. The MHD simulation is more compact than the hydrodynamic (HD) equivalent, with a less smooth distribution of gas. The star formation activity is also less spread-out, occurring mostly in the densest regions. The MHD galaxy also uniquely features a diffuse, atomic envelope above and below the plane. It has a higher fraction of dense gas than its HD counterpart, but a significantly lower star formation rate, at approximately 5.5 solar masses per year, in comparison to approximately 8.5 solar masses per year in the HD case. The headline result of this chapter is the characteristic shift of the MHD galaxy in the Kennicutt-Schmidt star formation relation to higher gas surface densities. In Chapter 4, with the same simulations, the effect of magnetic fields on the correlation between the atomic (HI) and molecular (H2) gas is studied. The HD model is found to have a higher correlation on 4pc scales, however, the presence of dense extended clumps inhibit the analysis. This will be revisited using simulations with smaller sink particles, as the extended features dissipate at higher resolution. In Chapter 5, a new galaxy model is presented, with sink particles reduced in size from 5pc down to 2pc. This allows the alignment between the magnetic field and the gas to be studied. The results show the gas becomes less aligned with the field as density increases, and more aligned as field strength increases, with decreasing galactic radii. Overall, it is clear that realistic magnetic fields have a distinct impact on the ISM and star formation. Gas is required to reach higher densities in the presence of magnetic fields to overcome the additional pressure and trigger collapse. Furthermore, magnetic fields affect the flow of gas in galaxies, setting preferential directions, and leading to structures with specific orientations.

Date of Award	6 Jan 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Gary Fuller (Supervisor) & Rowan Smith (Supervisor)