Magnetic Spiral Arms and Galactic Outflows

TL;DR

This paper proposes that galactic outflows influence magnetic arm formation, leading to stronger magnetic fields in interarm regions, and models how spiral structure affects magnetic dynamo processes in galaxies.

Contribution

It introduces a model where galactic outflows generate magnetic arms in interarm regions and explores their impact on dynamo saturation levels.

Findings

Magnetic arms can naturally form in interarm regions due to weaker outflows.

Outflows help prevent dynamo quenching by removing small-scale magnetic helicity.

Interarm regions exhibit higher large-scale magnetic field strengths than gaseous arms.

Abstract

Galactic magnetic arms have been observed between the gaseous arms of some spiral galaxies; their origin remains unclear. We suggest that magnetic spiral arms can be naturally generated in the interarm regions because the galactic fountain flow or wind is likely to be weaker there than in the arms. Galactic outflows lead to two countervailing effects: removal of small-scale magnetic helicity, which helps to avert catastrophic quenching of the dynamo, and advection of the large-scale magnetic field, which suppresses dynamo action. For realistic galactic parameters, the net consequence of outflows being stronger in the gaseous arms is higher saturation large-scale field strengths in the interarm regions as compared to in the arms. By incorporating rather realistic models of spiral structure and evolution into our dynamo models, an interlaced pattern of magnetic and gaseous arms can be produced.