Magnetic destabilisation in disc galaxies: Filament/feather formation

Abstract

Gas gravitational instability plays a crucial role in secular galactic evolution, but the role played by magnetic fields in mediating this instability - despite their dynamical relevance and ubiquity - is still not understood. To investigate this question we conduct a parameter study using numerical simulations of 3D isolated disc galaxies that are initialized in equilibrium, but have a range of initial magnetisation, quantified by $β\in \{0.1, 0.5, 1, 10, 100, \infty\}$, where $β$ is the ratio of thermal to magnetic pressure. We analyse how magnetic field strength influences the formation of dense filaments and feathers driven by gravitational instability. The simulations show that filament growth rate and spacing are significantly altered by dynamically strong fields ($β\lesssim 10$), and that these effects depend on the value of $β$ and strength of shear in the disc. Magnetic fields either stabilise or destabilise, with destabilisation dominating in regions with low $β$, and low shear. This makes the destabilisation particularly important in dwarf galaxies with low-shear rotation curves. Filament spacings are similarly affected differently in different galactic regions, depending upon the local field strength and shear. Our results are in good agreement with predictions from the magneto-Jeans mechanism.