Star Formation and Magnetic Field Amplification due to Galactic Spirals

We use global MHD galaxy simulations to investigate the effects of spiral arms on the evolution of magnetic fields and star formation within a self-regulated interstellar medium. The same galaxy is simulated twice: once with self-consistent stellar spiral arms and once more with the stellar spirals suppressed via a novel numerical approach, using the Ramses adaptive mesh refinement code. Spiral arms continually promote star formation, with 2.6 times higher rates in the spiral galaxy. The higher rate is due to high gas columns gathered along the spiral arms, rather than increasing the star formation efficiency at a given gas column. In both cases, the magnetic field is initially amplified via a small-scale dynamo driven by turbulence due to supernova feedback. Only the spiral galaxy exhibits late-time, consistent field growth due to a large-scale dynamo (e-folding time ∼600 Myr). This results in volume-averaged field strengths of ∼1 μG after 1 Gyr of evolution. The mean fields tend to align themselves with the spiral arms and are coherent up to 10 kpc scales. We demonstrate a novel large-scale dynamo mechanism, whereby spiral-driven radial flows enable the mean-field amplification.