A bottleneck for star formation: the importance of magnetic fields during the formation of cold gas in galaxies

TL;DR

This study investigates how magnetic fields influence the formation of cold gas in galaxies by conducting a high-resolution, parsec-scale magnetohydrodynamic simulation of a dwarf galaxy with non-equilibrium chemistry. By comparing magnetic, thermal, kinetic, and self-gravitating energies per gas cell, the authors show that while thermal energy dominates most of the ISM, magnetic energy becomes significant in the thermally unstable regime and the cold neutral medium, and similarly substantial in molecular gas (including CO-dark gas). The results suggest magnetic pressures slow the collapse toward dense, star-forming gas, effectively acting as a bottleneck that reshapes the cold-gas reservoir and, consequently, star-formation efficiency. This magnetic regulation provides a natural explanation for extended cold gas around star-forming clouds and aligns with observed gas-filament orientations relative to magnetic fields, highlighting the integral role of magnetic fields in the baryon cycle of galaxies.

Abstract

Using a high-resolution simulation of a dwarf galaxy, we quantify the energetic importance of magnetic fields within the different phases of its interstellar medium (ISM) on parsec scales. We show that, whilst overall the magnetic field is only energetically dominant for a small fraction of the ISM, it becomes important in the thermally unstable regime (45.2% of the mass is magnetically dominated), and in the majority of the cold neutral medium (66.1% of the mass). In the molecular gas, the magnetic field dominates more of the total mass budget (39.8%) than thermal energy, turbulent kinetic energy, or gas self-gravitating potential energy. However, much of this gas will be CO-dark. This suggests that magnetic forces are non-negligible during the formation of cold dense gas, which will slow its collapse and lead to an increase in the fraction of cold atomic, and molecular gas in the ISM. Consequently, star-forming clouds may be surrounded by a larger reservoir of cold gas than would otherwise be expected.

Figures (16)

• Figure 1: Projection of the HI and H$_2$ surface density for our fiducial model. The molecular gas chiefly lies in the inner galaxy.
• Figure 2: The star formation rate and volume weighted magnetic field strength of our fiducial model MHD_SOL (blue) over the steady state period, as described in Whitworth2023. The model has reached a steady star formation rate at the time of analysis (1 Gyr). A second low metallicity model (MHD_SAT in red) is shown for later comparison in Section \ref{['sec:metallicity']}.
• Figure 3: Cell radius as a function of number density for our fiducial model, where the radius is calculated as that of a sphere of the same volume as the cell. All logs are in base 10. The resolution is sub-parsec for all densities greater than 100 cm$^{-3}$.
• Figure 4: Mass-weighted absolute magnitude of the magnetic field vs the gas number density for at 1 Gyr for the solar metallicity model. The dotted black lines indicate the Crutcher2010 relation, with and without a flattening at low density. The solid black line shows the slope derived in Whitworth2025b.
• Figure 5: Radial profiles of the cylindrical components of the velocity dispersion for neutral, ionised and molecular hydrogen as well as CO.