Magnetic field structure due to the global velocity field in spiral galaxies

TL;DR

This study uses advanced N-body/SPH simulations to explore how magnetic fields evolve and align with spiral structures in galactic discs, highlighting the role of non-axisymmetry and resolution in magnetic amplification.

Contribution

It introduces a self-consistent simulation approach for magnetic field evolution in galactic discs, comparing different numerical methods and demonstrating magnetic amplification and alignment with spiral patterns.

Findings

Magnetic fields are amplified by differential rotation and spiral formation within five disc rotations.

Higher numerical resolution results in stronger magnetic field amplification.

Magnetic field lines align with the galaxy's spiral density pattern.

Abstract

We present a set of global, self-consistent N-body/SPH simulations of the dynamic evolution of galactic discs with gas and including magnetic fields. We have implemented a description to follow the evolution of magnetic fields with the ideal induction equation in the SPH part of the Vine code. Results from a direct implementation of the field equations are compared to a representation by Euler potentials, which pose a div(B)-free description, an constraint not fulfilled for the direct implementation. All simulations are compared to an implementation of magnetic fields in the Gadget code which includes also cleaning methods for div(B). Starting with a homogeneous seed field we find that by differential rotation and spiral structure formation of the disc the field is amplified by one order of magnitude within five rotation periods of the disc. The amplification is stronger for higher numerical resolution. Moreover, we find a tight connection of the magnetic field structure to the density pattern of the galaxy in our simulations, with the magnetic field lines being aligned with the developing spiral pattern of the gas. Our simulations clearly show the importance of non-axisymmetry for the evolution of the magnetic field.