Magnetic field evolution in dwarf and Magellanic-type galaxies

TL;DR

This study models how cosmic-ray-driven dynamo processes can amplify magnetic fields in low-mass dwarf and Magellanic-type galaxies, showing that starburst events temporarily boost magnetic strength and influence gas expulsion.

Contribution

It demonstrates that cosmic-ray-driven dynamo can effectively amplify magnetic fields in low-mass galaxies, a process not primarily driven by large-scale dynamo mechanisms.

Findings

Magnetic field amplification occurs within 0.25-0.50 Gyr in dwarf galaxies.

Starburst events temporarily enhance magnetic field strength.

Starbursts increase gas expulsion from the galactic disk.

Abstract

Low-mass galaxies radio observations show in many cases surprisingly high levels of magnetic field. The mass and kinematics of such objects do not favour the development of effective large-scale dynamo action. We attempted to check if the cosmic-ray-driven dynamo can be responsible for measured magnetization in this class of poorly investigated objects. We investigated how starburst events on the whole, as well as when part of the galactic disk, influence the magnetic field evolution. We created a model of a dwarf/Magellanic-type galaxy described by gravitational potential constituted from two components: the stars and the dark-matter halo. The model is evolved by solving a three-dimensional (3D) magnetohydrodynamic equation with an additional cosmic-ray component, which is approximated as a fluid. The turbulence is generated in the system via supernova explosions manifested by the injection of cosmic-rays.The cosmic-ray-driven dynamo works efficiently enough to amplify the magnetic field even in low-mass dwarf/Magellanic-type galaxies. The $e$-folding times of magnetic energy growth are 0.50 and 0.25 Gyr for the slow (50 km/s) and fast (100 km/s) rotators, respectively. The amplification is being suppressed as the system reaches the equipartition level between kinetic, magnetic, and cosmic-ray energies. An episode of star formation burst amplifies the magnetic field but only for a short time while increased star formation activity holds. We find that a substantial amount of gas is expelled from the galactic disk, and that the starburst events increase the efficiency of this process.