Next Article in Journal
Constraints to Dark Matter Annihilation from High-Latitude HAWC Unidentified Sources
Previous Article in Journal
ConvoSource: Radio-Astronomical Source-Finding with Convolutional Neural Networks
Previous Article in Special Issue
The Origin of Large-Scale Magnetic Fields in Low-Mass Galaxies
Open AccessReview

Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

1
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
3
Department of Physics, University of the Western Cape, Belleville 7535, South Africa
*
Author to whom correspondence should be addressed.
Galaxies 2020, 8(1), 4; https://doi.org/10.3390/galaxies8010004
Received: 10 October 2019 / Revised: 13 December 2019 / Accepted: 16 December 2019 / Published: 21 December 2019
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
Constraining dynamo theories of magnetic field origin by observation is indispensable but challenging, in part because the basic quantities measured by observers and predicted by modelers are different. We clarify these differences and sketch out ways to bridge the divide. Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies. We consistently compute strengths of total, ordered, and regular fields, pitch angles of ordered and regular fields, and we summarize the present knowledge on azimuthal modes, field parities, and the properties of non-axisymmetric spiral features called magnetic arms. We review related aspects of dynamo theory, with a focus on mean-field models and their predictions for large-scale magnetic fields in galactic discs and halos. Furthermore, we measure the velocity dispersion of H i gas in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6946, since spiral modulation of the root-mean-square turbulent speed has been proposed as a driver of non-axisymmetry in large-scale dynamos. We find no evidence for such a modulation and place upper limits on its strength, helping to narrow down the list of mechanisms to explain magnetic arms. Successes and remaining challenges of dynamo models with respect to explaining observations are briefly summarized, and possible strategies are suggested. With new instruments like the Square Kilometre Array (SKA), large data sets of magnetic and non-magnetic properties from thousands of galaxies will become available, to be compared with theory. View Full-Text
Keywords: galaxies: magnetic fields; galaxies: kinematics and dynamics; radio continuum: galaxies; MHD; radio lines: galaxies; galaxies: spiral; galaxies: structure; galaxies: ISM galaxies: magnetic fields; galaxies: kinematics and dynamics; radio continuum: galaxies; MHD; radio lines: galaxies; galaxies: spiral; galaxies: structure; galaxies: ISM
Show Figures

Figure 1

MDPI and ACS Style

Beck, R.; Chamandy, L.; Elson, E.; Blackman, E.G. Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges. Galaxies 2020, 8, 4.

Show more citation formats Show less citations formats
Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here.

Article Access Map by Country/Region

1
Back to TopTop