PI: Rowan Smith
- McGuiness, Smith & Whitworth, 2026, MNRAS, accepted (10.48550/arXiv.2512.04184)
- Bogue et al. 2026, MNRAS, Volume 545, Issue 3
The formation of stars, and the evolution of galaxies are both controlled by the continuous cycle of gas between different states within the interstellar medium (ISM). Gas transitions from hot to warm to cold gas where stars form. The feedback from these stars then injects energy and momentum into their natal gas clouds, destroying them and restarting the process. However, our understanding of the formation of cold molecular clouds themselves is less developed, particularly in the context of magnetic fields. We used DiRAC to simulate extremely high-resolution models of galaxies, which included both magnetic fields and star formation Physics.
Figure 1: Left a dwarf galaxy simulated including magnetic fields showing the location of molecular and atomic hydrogen. Right a pie chart showing how much of the molecular gas is dominated by each force. Magnetic fields are the most significant force. McGuiness et al. 2026
We found that magnetic fields were energetically important during the formation of the dense molecular gas clouds that give birth to stars (Figure 1). This acted as a bottleneck, slowing down the formation of the clouds and delaying star formation. As a result, the structure of the gas disc was changed, and far more molecular gas was formed than would otherwise be the case. This also has implications for the Kennicutt-Schmidt relation that describes the gas density at which stars form in galaxies. Figure 2 shows how a spiral galaxy simulated with magnetic fields (MHD) is far more compact than the same model without the fields (HD). The stars then form at higher densities as they must overcome the magnetic pressure. This puts the model more in line with the observations, showing that magnetic forces are not negligible for determining where stars form in galaxies.
Figure 2: Left a spiral galaxy simulated with and without magnetic fields. Blue dots show where stars form. Right where each model would lie of the Kennicutt-Schmidt star formation relation. Magnetic fields make the galaxy more compact, and stars form at higher densities, which puts the simulations in better agreement with observations. Bogue et al. 2026