Global galactic dynamo driven by cosmic-rays and exploding magnetized stars

TL;DR

This paper presents pioneering global simulations of a cosmic-ray-driven galactic dynamo, showing how small-scale magnetic fields from supernovae can be amplified and organized into large-scale galactic magnetic structures.

Contribution

It introduces the first global galactic-scale CR-MHD simulations demonstrating the cosmic-ray-driven dynamo's role in magnetic field amplification and structure formation.

Findings

Magnetic fields are exponentially amplified to equipartition levels.

Small-scale dipolar fields evolve into large-scale spiral and X-shaped structures.

Cosmic rays facilitate the efficient dynamo process.

Abstract

We report first results of first global galactic-scale CR-MHD simulations of cosmic-ray-driven dynamo. We investigate the dynamics of magnetized interstellar medium (ISM), which is dynamically coupled with the cosmic-ray (CR) gas. We assume that exploding stars deposit small-scale, randomly oriented, dipolar magnetic fields into the differentially rotating ISM, together with a portion of cosmic rays, accelerated in supernova shocks. We conduct numerical simulations with the aid of a new parallel MHD code PIERNIK. We find that the initial magnetization of galactic disks by exploding magnetized stars forms a favourable conditions for the cosmic-ray-driven dynamo. We demonstrate that dipolar magnetic fields supplied on small SN-remnant scales, can be amplified exponentially, by the CR-driven dynamo, to the present equipartition values, and transformed simultaneously to large galactic-scales. The resulting magnetic field structure in an evolved galaxy appears spiral in the face-on view and reveals the so called X-shaped structure in the edge-on view.