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

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 per cent of the mass is magnetically dominant), and dominates in the cold neutral medium (66.1 per cent of the mass). In the molecular gas, the magnetic field dominates more of the total mass budget (39.8 per cent) than thermal energy, turbulent kinetic energy, or gas self-gravitating potential energy. However, much of this mass 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.