The Origin of Large-Scale Magnetic Fields in Low-Mass Galaxies

Prasanta Bera,Anvar Shukurov,Kandaswamy Subramanian

doi:10.3390/galaxies7040091

Abstract

The origin of large-scale magnetic fields, detected in some low-mass (dwarf and irregular) galaxies via polarised synchrotron emission and Faraday rotation, has remained unexplained for a long time. We suggest that mean-field dynamos can be active in galaxies of this class despite their slow rotation because their discs are relatively thick. Earlier assessments of the possibility of the mean-field dynamo action in low-mass galaxies relied on estimates applicable to thin discs, such as those in massive spiral galaxies. Using both order-of-magnitude estimates and numerical solutions, we show that the strength of differential rotation required to amplify magnetic field reduces as the aspect ratio of the galactic gas layer increases. As in a thin disc, quadrupolar magnetic fields dominate in thick discs. Thus, the origin of large-scale magnetic fields in low-mass galaxies has been clarified. This class of galaxies provides a new ground for testing our understanding of galactic magnetism.

Highlights

Large-scale magnetic fields are a prominent and widespread in disc galaxies [1,2,3]
Our goal was to clarify conditions required for mean-field dynamo action in the thick discs of low-mass galaxies rather than to explore magnetic field forms and distributions in specific galaxies
We focused on finding the critical values of the dynamo number and the corresponding rotational speed and only briefly discuss steady-state magnetic field distributions

Summary

Introduction

Large-scale magnetic fields are a prominent and widespread in disc galaxies [1,2,3]. Their origin is most likely due to the mean-field dynamo action that relies on random (turbulent) motions of the partially ionised interstellar gas, galactic differential rotation, and stratification of the gas distribution in the galactic gravity field. The mean-field dynamo theory (for a review, see [4]) explains many salient properties of galactic magnetic fields [5] (for a compendium of basic results, see [6]). Statistical comparisons of theory and observations of galactic magnetic fields can be found in Van Eck et al [9] and Chamandy et al [10].

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Conclusion