Simulations Exploring Magnetic Fields in Spiral Galaxies

Abstract

Magnetism affects all scales in astrophysics because regular matter in the Universe is mostly ionized and couples to magnetic fields. Galaxy evolution entails many unanswered questions surrounding magnetic fields, including how they affect the flow and distribution of gas within the interstellar medium, how they affect the process of star formation, and how they evolve in a galactic environment. We investigate these questions through a series of isolated galaxy magnetohydrodynamic (MHD) simulations, which include additional physics such as stellar feedback, star formation, gas cooling and turbulence. To isolate the effects of the magnetic fields we use controlled setups that vary only the spiral nature of the galaxy and its initial magnetic fields.

We find that galaxies simulated with stronger magnetic fields have increased disk stability and reduced star formation rates compared to those without magnetic fields. Magnetic fields provide additional pressure that increases the scale height of the galactic disk. Secondly, we demonstrate the presence of several dynamos, including a novel galactic dynamo which is enabled by the presence of non-axisymmetric features such as bars and spiral arms. By simulating the same galaxy with and without spiral arms, we show that the galaxies can generate large-scale azimuthal fields whose amplification depends upon radial flows of gas. The presence of spiral arms also increases the overall amount of star formation in the galaxies, which in turn generates stronger fields through the turbulent dynamo. We show that the initial configuration of the fields does not affect the overall star formation rate, but does impact the transport of angular momentum, which can inhibit or promote the formation of a central bar and associated central star formation and field amplification.