The origin of the structure of large-scale magnetic fields in disc galaxies

TL;DR

This paper proposes a model where vertical shear acting on an initial poloidal magnetic field explains the observed large-scale spiral magnetic structures in disc galaxies, emphasizing the role of the WIM/halo interface.

Contribution

It introduces a simple model attributing galactic magnetic spiral patterns to vertical shear effects on poloidal fields, challenging traditional dynamo-centric views.

Findings

Model reproduces main observed properties of galactic magnetic fields.

Vertical shear can generate spiral polarization patterns from poloidal fields.

Highlights the significance of the WIM/halo interface in magnetic field observations.

Abstract

The large-scale magnetic fields observed in spiral disc galaxies are often thought to result from dynamo action in the disc plane. However, the increasing importance of Faraday depolarization along any line of sight towards the galactic plane suggests that the strongest polarization signal may come from well above (~0.3-1 kpc) this plane, from the vicinity of the warm interstellar medium (WIM)/halo interface. We propose (see also Henriksen & Irwin 2016) that the observed spiral fields (polarization patterns) result from the action of vertical shear on an initially poloidal field. We show that this simple model accounts for the main observed properties of large-scale fields. We speculate as to how current models of optical spiral structure may generate the observed arm/interarm spiral polarization patterns.