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

TL;DR

This study investigates how magnetic fields influence cold, star-forming gas in galaxies by running two Arepo simulations of an isolated disc galaxy: one with magnetic fields (MHD) and one purely hydrodynamic (HD). Unlike many galaxy-scale studies, star formation arises from gravitationally bound gas via sink particles, allowing the Kennicutt-Schmidt (KS) relation to emerge self-consistently, while a time-dependent chemical network and SN feedback produce a realistic multi-phase ISM. The MHD run forms a more compact disc with a diffuse halo, amplifies the magnetic field through dynamo action to a few microgauss, and exhibits a lower SFR ($\sim4.8\,M_\odot\,\mathrm{yr}^{-1}$) compared with the HD case ($\sim8.4\,M_\odot\,\mathrm{yr}^{-1}$). The KS relation shifts to higher gas surface densities in the presence of magnetic fields, indicating that magnetic support raises the density threshold for star formation and brings the simulated system into closer agreement with observations, highlighting the important role of magnetic dynamics in galaxy-scale star formation.

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.

Figures (18)

• Figure 1: Variation in cell size with gas density for the MHD model.
• Figure 2: Face-on total gas column density $N_{\rm tot}$ for the MHD simulation at $\sim$ 25 Myr intervals, illustrating the morphological evolution over 225 Myr. These projections are column integrals along sightlines for each projected pixel, calculated by ray tracing through the Voronoi grid.
• Figure 3: Face-on projections of the absolute magnetic field strength, $|B| = (B_x^2 + B_y^2 + B_z^2)^{1/2}$, calculated as mass-weighted column integrals, shown for the same intervals as in Figure \ref{['fig:MHD_col_grid_KSP']}. In this way, the gas column density and absolute magnetic field strengths across the disc can be visually compared.
• Figure 4: Face-on total gas column density $N_{\rm tot}$ for the HD simulation, for the same time intervals as Figure \ref{['fig:MHD_col_grid_KSP']}.
• Figure 5: Total gas mass of the disc of the MHD (solid blue) and HD (solid red) models over time. The grey region denotes the initialisation phase, during which the galaxies are still responding to their initial condition. We see a gradual depletion in mass, mostly resulting from star formation. We additionally plot the combined mass of gas cells and the gas in sink particles, for both the MHD (dotted blue line) and HD (dotted red line) models. As gas is converted into stars, and there is little to no gas in-flow to the disc of our isolated galaxies, the gas reservoir is eventually depleted.