| Abstract
| - 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 ∝ ρκ. 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 H 2 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.
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