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A physical approach to modelling large-scale galactic magnetic fields

Lookup NU author(s): Professor Anvar Shukurov, Dr Luiz Felippe Rodrigues, Dr Paul Bushby, James Hollins

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This is the authors' accepted manuscript of an article that has been published in its final definitive form by EDP Sciences, 2019.

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Abstract

Context. A convenient representation of the structure of the large-scale galactic magnetic field is required for the interpretation of polarization data in the sub-mm and radio ranges, in both the Milky Way and external galaxies. Aims. We develop a simple and flexible approach to construct parametrised models of the large-scale magnetic field of the Milky Way and other disc galaxies, based on physically justifiable models of magnetic field structure. The resulting models are designed to be optimised against available observational data. Methods. Representations for the large-scale magnetic fields in the flared disc and spherical halo of a disc galaxy were obtained in the form of series expansions whose coefficients can be calculated from observable or theoretically known galactic properties. The functional basis for the expansions is derived as eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic diffusion equation. Results. The solutions presented are axially symmetric but the approach can be extended straightforwardly to non-axisymmetric cases. The magnetic fields are solenoidal by construction, can be helical, and are parametrised in terms of observable properties of the host object, such as the rotation curve and the shape of the gaseous disc. The magnetic field in the disc can have a prescribed number of field reversals at any specified radii. Both the disc and halo magnetic fields can separately have either dipolar or quadrupolar symmetry. The model is implemented as a publicly available software package galmag which allows, in particular, the computation of the synchrotron emission and Faraday rotation produced by the model’s magnetic field. Conclusions. The model can be used in interpretations of observations of magnetic fields in the Milky Way and other spiral galaxies, in particular as a prior in Bayesian analyses. It can also be used for a simple simulation of a time-dependent magnetic field generated by dynamo action.


Publication metadata

Author(s): Shukurov A, Rodrigues LFS, Bushby PJ, Hollins J, Rachen JP

Publication type: Article

Publication status: Published

Journal: Astronomy & Astrophysics

Year: 2019

Volume: 623

Print publication date: 13/03/2019

Online publication date: 28/01/2019

Acceptance date: 22/01/2019

Date deposited: 11/02/2019

ISSN (print): 0004-6361

ISSN (electronic): 1432-0746

Publisher: EDP Sciences

URL: https://doi.org/10.1051/0004-6361/201834642

DOI: 10.1051/0004-6361/201834642


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