2024A&A...686A...8K

Context. One of the indicators most frequently used to characterize the magnetic field's influence on star formation is the relation between the magnetic field strength and the gas density (the B - p relation), usually expressed as a power law of the form B ∝ ρ^κ^. The value of κ is an indication of the dynamical importance of the magnetic field during gas compression.
Aims. In this work, we investigate the role of the global magnetic field morphology on a galaxy's B - ρ relation, as well as the evolution of the relation over time.
Methods. We developed magnetohydrodynamic simulations of Milky Way-like galaxies that include gravity, star formation, and supernova feedback. The models take into account nonequilibrium chemistry up to H₂ formation, which is used to fuel star formation. We considered two different initial magnetic field morphologies: one completely ordered (toroidal) and the other completely random. Using these models, we studied the dynamical importance of the magnetic field through the plasma ß and the B - ρ relation.
Results. For both magnetic morphologies, low-density regions are thermally supported, while high-density regions are magnetically dominated. Equipartition is reached earlier and at lower densities in the toroidal model. However, the B - ρ relation varies, even within the same galaxy, as it consistently includes two different branches for a given density, with κ ranging from about 0.2 to 0.8. The mean value of κ for each model also varies significantly over time, which supersedes the differences between the two models.
Conclusions. While our findings suggest that the magnetic field morphology does influence the galactic B - ρ relation, its impact is transient in nature since time-averaged differences between the models fall within the large temporal scatter. The context and time-dependent nature of the B - ρ relation underscore the need for comprehensive research and observations to understand the intricate role of magnetic fields in star formation processes across diverse galactic environments.