Cosmic-ray driven dynamo in the interstellar medium of irregular galaxies

TL;DR

This study demonstrates that cosmic-ray driven dynamos can efficiently amplify magnetic fields in irregular galaxies despite slow rotation and weak shear, potentially contributing to intergalactic magnetic fields.

Contribution

It introduces a numerical magnetohydrodynamical model of the cosmic-ray driven dynamo tailored for irregular galaxies with slow rotation and low shear.

Findings

Magnetic fields are amplified even with slow galactic rotation.

Rapid rotation and low shear increase dynamo efficiency.

A significant portion of magnetic energy escapes, seeding intergalactic space.

Abstract

Irregular galaxies are usually smaller and less massive than their spiral, S0, and elliptical counterparts. Radio observations indicate that a magnetic field is present in irregular galaxies whose value is similar to that in spiral galaxies. However, the conditions in the interstellar medium of an irregular galaxy are unfavorable for amplification of the magnetic field because of the slow rotation and low shearing rate. We investigate the cosmic-ray driven dynamo in the interstellar medium of an irregular galaxy. We study its efficiency under the conditions of slow rotation and weak shear. The star formation is also taken into account in our model and is parametrized by the frequency of explosions and modulations of activity. The numerical model includes a magnetohydrodynamical dynamo driven by cosmic rays that is injected into the interstellar medium by randomly exploding supernovae. In the model, we also include essential elements such as vertical gravity of the disk, differential rotation approximated by the shearing box, and resistivity leading to magnetic reconnection. We find that even slow galactic rotation with a low shearing rate amplifies the magnetic field, and that rapid rotation with a low value of the shear enhances the efficiency of the dynamo. Our simulations have shown that a high amount of magnetic energy leaves the simulation box becoming an efficient source of intergalactic magnetic fields.