The role of magnetic fields in disc galaxies: spiral arm instability

Context. Regularly-spaced, star-forming regions along the spiral arms of nearby galaxies provide insight into the early stages and initial conditions of star formation. The regular separation of these star-forming regions suggests spiral arm instability as their origin. Aims. We explore the effects of magnetic fields on the spiral arm instability. Methods. We use three-dimensional global magnetohydrodynamical simulations of isolated spiral galaxies, comparing three different initial plasma $\beta$ values (ratios of thermal to magnetic pressure) of $\beta=\infty$, $50$, and $10$. We perform Fourier analysis to calculate the separation of the over-dense regions formed from the spiral instability. We then compare the separations with observations. Results. We find that the spiral arms in the hydro case ($\beta = \infty$) are unstable, with the fragments initially connected by gas streams, reminiscent of Kelvin-Helmholtz instability. In the $\beta = 50$ case, the spiral arms fragment, but the fragments separate earlier and tend to be elongated in the direction perpendicular to the spiral arms. However, in the $\beta = 10$ run the arms are stabilised against fragmentation by magnetic pressure. The spiral arms in the unstable cases fragment into regularly-spaced, over-dense regions. We determine their separation to be $\sim 0.5$ kpc in the hydro and $\sim 0.65$ kpc in the $\beta = 50$ case, both in agreement with the observations of nearby galaxies. We find a smaller median characteristic wavelength of the over-densities to be $0.73^{+0.31}_{-0.36}$ kpc in the hydro case, compared to $0.98^{+0.49}_{-0.46}$ kpc in the $\beta = 50$ case. Moreover, we find a higher growth rate of the over-densities in the $\beta = 50$ run compared to the hydro run. We observe magnetic hills and valleys along the fragmented arms in the $\beta = 50$ run, which is characteristic of the Parker instability.