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Open AccessArticle

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

by Prasanta Bera 1,2,3,*,†, Anvar Shukurov 4,*,† and Kandaswamy Subramanian 1,†
1
Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Pune 411 007, India
2
National Centre for Radio Astrophysics, TIFR, Post Bag 3, Pune 411 007, India
3
Mathematical Sciences and STAG Research Centre, University of Southampton, Southampton SO17 1BJ, UK
4
School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
*
Authors to whom correspondence should be addressed.
These authors contributed equally to this work.
Galaxies 2019, 7(4), 91; https://doi.org/10.3390/galaxies7040091
Received: 14 October 2019 / Revised: 20 November 2019 / Accepted: 26 November 2019 / Published: 29 November 2019
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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. View Full-Text
Keywords: dwarf galaxies; irregular galaxies; interstellar medium; magnetic fields; dynamo; radio continuum dwarf galaxies; irregular galaxies; interstellar medium; magnetic fields; dynamo; radio continuum
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Bera, P.; Shukurov, A.; Subramanian, K. The Origin of Large-Scale Magnetic Fields in Low-Mass Galaxies. Galaxies 2019, 7, 91.

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