Time evolution of the galactic B - ρ relation : the impact of the magnetic field morphology

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 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.