Role of magnetic fields in disc galaxies: spiral arm instability

Abstract

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 3D global magnetohydrodynamical simulations of isolated spiral galaxies, comparing three different initial plasma β values (ratios of the thermal to magnetic pressure) of β = ∞, 50, and 10. We perform a Fourier analysis to calculate the separation of the over-dense regions that formed as a result of the spiral instability. We then compare the separations with observations. Results. We find that the spiral arms in the hydro case (β = ∞) are unstable. The fragments are initially connected by gas streams that are reminiscent of the Kelvin-Helmholtz instability. For β = 50, the spiral arms also fragment, but the fragments separate earlier and tend to be slightly elongated in the direction perpendicular to the spiral arms. However, in the β = 10 run, the arms are stabilised against fragmentation by magnetic pressure. Despite the difference in the initial magnetic field strengths of the β = 50 and 10 runs, the magnetic field is amplified to βarm ∼ 1 inside the spiral arms for both runs. The spiral arms in the unstable cases (hydro and β = 50) fragment into regularly spaced over-dense regions. We determine their separation to be ∼0.5 kpc in the hydro and ∼0.65 kpc in the β = 50 case. These two values agree with the observed values found in nearby galaxies. We find a smaller median characteristic wavelength of the over-densities along the spiral arms of 0.73−0.36+0.31 kpc in the hydro case compared to 0.98−0.46+0.49 kpc in the β = 50 case. Moreover, we find a higher growth rate of the over-densities in the β = 50 run compared to the hydro run. We observe magnetic hills and valleys along the fragmented arms in the β = 50 run, which is characteristic of the Parker instability.