Abstract
We present a set of global, self-consistent N-body/smoothed particle hydrodynamic (SPH) simulations of the dynamic evolution of galactic discs with gas, 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 ∇·B-free description, a constraint not fulfilled for the direct implementation. All simulations are compared to an implementation of magnetic fields in the gadget code which also includes cleaning methods for ∇·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.
Highlights
Radio observations have revealed that disc galaxies are permeated by large scale magnetic fields ordered on kpc scales and beyond (Beck & Hoernes 1996, Hummel & Beck 1995, Beck et al 1985)
This difference is probably caused by the numerical ∇ · B in these simulations (Fig. 10), but possibly by the fact that field winding is not traced beyond a certain evolutionary state in the Euler potentials formulation
Since the Euler potentials are free from physical divergence by construction, the numerical divergence in simulations using the Euler potentials is due to the smooth particle hydrodynamic (SPH) derivative approximation when calculating the magnetic field from the potentials (Eq 18)
Summary
Radio observations have revealed that disc galaxies are permeated by large scale magnetic fields ordered on kpc scales and beyond (Beck & Hoernes 1996, Hummel & Beck 1995, Beck et al 1985). It reveals the tight connection of magnetic field with the gas distribution in the galactic disc. The motion of the gas within the gravitational potential of a galaxy strongly influences the strength and direction of the magnetic field in the interstellar medium. This can be seen by inspecting the well known induction equation of magnetohydrodynamics (MHD), i.e. the temporal evolution
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