Magnetic Field Alignment in Galaxy Simulations

Sammanfattning

Spiral galaxies host elongated filamentary structures of dense gas and dust, known as feathers, which branch off the main spiral arms into the interarm regions and are associated with star formation. The physical mechanisms governing their formation remain debated, with magnetic fields proposed as one of the contributing factors. This thesis investigates the role of magnetic fields in feather formation by analysing isolated galaxy simulations of self-gravitating, isothermal, magnetised discs. Two simulation runs are compared, differing only in the strength of the initial toroidal magnetic field: a strongly magnetised case at equipartition (β = 1) and a weakly magnetised case at sub-equipartition (β = 10). The geometric relationship between the gas structures and the local magnetic field is quantified using the Histogram of Relative Orientations (HRO), which gives a statistical measure of the alignment between the gas structures and the magnetic field as a function of log-normalised projected surface density. The simulations reveal that in the strongly magnetised regime, the transition from parallel to perpendicular alignment between the gas structures and the magnetic field begins already at lower densities, well before the feather formation threshold is reached. In the weakly magnetised regime, this transition occurs much later, close to the feather formation threshold itself. Beyond this threshold, both regimes converge to predominantly perpendicular alignment, with the dense feather structures forming preferentially across the field lines rather than along them. However, the strongly magnetised case reaches a more negative value of the alignment parameter, indicating that this configuration is more dominant when the magnetic field is dynamically important. These results suggest that the density at which the alignment transition occurs may serve as an observational indicator of the dynamical importance of the magnetic field in galactic discs.