Special Issue "New Perspectives on Galactic Magnetism"

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: 15 December 2019.

Special Issue Editors

Guest Editor
Dr. Sui Ann Mao

Max Planck Institute for Radio Astronomy, 53121 Bonn, Germany
Website | E-Mail
Interests: origin and evolution of cosmic magnetism; interstellar medium; radio polarimetry; radio astronomy; galaxy evolution
Guest Editor
Dr. Andrew Fletcher

School of Mathematics and Statistics, Newcastle University, NE1 7RU, UK
Website | E-Mail
Interests: galactic magnetic fields; the interstellar medium; cosmic rays; galaxy evolution; radio astronomy; polarimetry

Special Issue Information

Dear Colleagues,

We invite you to submit manuscripts for a Special issue of Galaxies on “New Perspectives on Galactic Magnetism”. Rapid improvements in the observing capabilities of radio telescopes and in the physical realism of numerical simulations, as well as growing activities due to the Square Kilometre Array and its pathfinders, suggest to us that an overview of the current state of the field would be useful and timely. This Special Issue aims to provide a platform to facilitate effective communication between observers, theorists and simulators to advance our field in new directions. It will contain both critical reviews and current research articles on new developments in all aspects of galactic magnetism. We particularly welcome contributions on: The connection with other components of the interstellar medium, including cosmic rays; the use of numerical simulations to help interpret observations; revisiting and addressing common assumptions used in the field; new observational techniques and recent advances in numerical simulations.

Sincerely

Dr. Sui Ann Mao
Dr. Andrew Fletcher
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Galaxies is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • galactic magnetic fields
  • the interstellar medium
  • cosmic rays
  • galaxy evolution
  • radio astronomy
  • polarimetry
  • magneto-hydrodynamics simulations
  • dynamo theory

Published Papers (7 papers)

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Research

Jump to: Review

Open AccessArticle
Magnetic Fields and Halos in Spiral Galaxies
Received: 18 February 2019 / Revised: 14 April 2019 / Accepted: 18 April 2019 / Published: 4 May 2019
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Abstract
Radio continuum and polarization observations reveal best the magnetic field structure and strength in nearby spiral galaxies. They show a similar magnetic field pattern, which is of spiral shape along the disk plane and X-shaped in the halo, sometimes accompanied by strong vertical [...] Read more.
Radio continuum and polarization observations reveal best the magnetic field structure and strength in nearby spiral galaxies. They show a similar magnetic field pattern, which is of spiral shape along the disk plane and X-shaped in the halo, sometimes accompanied by strong vertical fields above and below the central region of the disk. The strength of the total halo field is comparable to that of the disk. The small- and large-scale dynamo action is discussed to explain the observations with special emphasis on the rôle of star formation on the α Ω dynamo and the magnetic field strength and structure in the disk and halo. Recently, with RM-synthesis of the CHANG-ES observations, we obtained the first observational evidence for the existence of regular magnetic fields in the halo. The analysis of the radio scale heights indicate escape-dominated radio halos with convective cosmic ray propagation for many galaxies. These galactic winds may be essential for an effective dynamo action and may transport large-scale magnetic field from the disk into the halo. Full article
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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Open AccessArticle
Solar and Galactic Magnetic Halo Structure: Force-Free Dynamos?
Received: 14 January 2019 / Revised: 5 April 2019 / Accepted: 18 April 2019 / Published: 3 May 2019
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Abstract
Magnetic fields may relax dissipatively to the minimum energy force-free condition whenever they are not constantly created or distorted. We review the axially symmetric solutions for force-free magnetic fields, especially for the non-linear field. A new formulation for the scale invariant state is [...] Read more.
Magnetic fields may relax dissipatively to the minimum energy force-free condition whenever they are not constantly created or distorted. We review the axially symmetric solutions for force-free magnetic fields, especially for the non-linear field. A new formulation for the scale invariant state is given. Illustrative examples are shown. Applications to both stellar coronas and galactic halos are possible. Subsequently we study whether such force-free fields may be sustained by classical magnetic dynamo action. Although the answer is `not indefinitely’, there may be an evolutionary cycle wherein the magnetic field repeatedly relaxes to the minimum energy condition after a period of substantial growth and distortion. Different force-free dynamos may coexist at different locations. Helicity transfer between scales is studied briefly. A dynamo solution is given for the temporal evolution away from an initial linear force-free magnetic field due to both α 2 and ω terms. This can be used at the sub scale level to create a `delayed’ α effect. Full article
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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Open AccessArticle
Faraday Rotation of Extended Emission as a Probe of the Large-Scale Galactic Magnetic Field
Received: 30 January 2019 / Revised: 14 March 2019 / Accepted: 22 March 2019 / Published: 27 March 2019
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Abstract
The Galactic magnetic field is an integral constituent of the interstellar medium (ISM), and knowledge of its structure is crucial to understanding Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources have been the basis of comprehensive Galactic magnetic field models. Polarised [...] Read more.
The Galactic magnetic field is an integral constituent of the interstellar medium (ISM), and knowledge of its structure is crucial to understanding Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources have been the basis of comprehensive Galactic magnetic field models. Polarised extended emission (XE) is also seen along lines of sight through the Galactic disk, and also displays the effects of Faraday rotation. Our aim is to investigate and understand the relationship between EG and XE RMs near the Galactic plane, and to determine how the XE RMs, a hitherto unused resource, can be used as a probe of the large-scale Galactic magnetic field. We used polarisation data from the Canadian Galactic Plane Survey (CGPS), observed near 1420 MHz with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope. We calculated RMs from a linear fit to the polarisation angles as a function of wavelength squared in four frequency channels, for both the EG sources and the XE. Across the CGPS area, 55 < < 193 , 3 < b < 5 , the RMs of the XE closely track the RMs of the EG sources, with XE RMs about half the value of EG-source RMs. The exceptions are places where large local HII complexes heavily depolarise more distant emission. We conclude that there is valuable information in the XE RM dataset. The factor of 2 between the two types of RM values is close to that expected from a Burn slab model of the ISM. This result indicates that, at least in the outer Galaxy, the EG and XE sources are likely probing similar depths, and that the Faraday rotating medium and the synchrotron emitting medium have similar variation with galactocentric distance. Full article
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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Open AccessArticle
Magnetic Fields Around Galactic Discs
Received: 25 January 2019 / Revised: 22 February 2019 / Accepted: 27 February 2019 / Published: 7 March 2019
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Abstract
Magnetic fields in the discs of spiral galaxies are quite well understood, although, of course, many details still require investigation and future observations with new generations of radio telescopes will be valuable here. Magnetic configurations around galactic discs and, in particular, the magnetic [...] Read more.
Magnetic fields in the discs of spiral galaxies are quite well understood, although, of course, many details still require investigation and future observations with new generations of radio telescopes will be valuable here. Magnetic configurations around galactic discs and, in particular, the magnetic field components perpendicular to galactic discs seem to be much more poorly understood and deserve further investigation both observationally and by modelling. Another problem to be addressed in future investigations is the magnetic configuration in galactic halos and, in particular, interactions with the intergalactic medium and various winds. Finally, the importance of the observational determination of such drivers of galactic dynamo action as mirror asymmetry of the turbulent galactic flows are briefly discussed. Full article
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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Review

Jump to: Research

Open AccessReview
Practical Modeling of Large-Scale Galactic Magnetic Fields: Status and Prospects
Received: 30 January 2019 / Revised: 19 April 2019 / Accepted: 23 April 2019 / Published: 28 April 2019
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Abstract
This is a review of the status of efforts to model the large-scale Galactic magnetic field (GMF). Though important for a variety of astrophysical processes, the GMF remains poorly understood despite some interesting new tracers being used in the field. Though we still [...] Read more.
This is a review of the status of efforts to model the large-scale Galactic magnetic field (GMF). Though important for a variety of astrophysical processes, the GMF remains poorly understood despite some interesting new tracers being used in the field. Though we still have too many models that might fit the data, this is not to say that the field has not developed in the last few years. In particular, surveys of polarized dust have given us a new observable that is complementary to the more traditional radio tracers, and a variety of other new tracers and related measurements are becoming available to improve current modeling. This paper reviews: the tracers available; the models that have been studied; what has been learned so far; what the caveats and outstanding issues are; and one opinion of where the most promising future avenues of exploration lie. Full article
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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Open AccessReview
From Primordial Seed Magnetic Fields to the Galactic Dynamo
Received: 7 March 2019 / Revised: 7 April 2019 / Accepted: 9 April 2019 / Published: 14 April 2019
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Abstract
The origin and maintenance of coherent magnetic fields in the Universe is reviewed with an emphasis on the possible challenges that arise in their theoretical understanding. We begin with the interesting possibility that magnetic fields originated at some level from the early universe. [...] Read more.
The origin and maintenance of coherent magnetic fields in the Universe is reviewed with an emphasis on the possible challenges that arise in their theoretical understanding. We begin with the interesting possibility that magnetic fields originated at some level from the early universe. This could be during inflation, the electroweak, or the quark-hadron phase transitions. These mechanisms can give rise to fields which could be strong, but often with much smaller coherence scales than galactic scales. Their subsequent turbulent decay decreases their strength but increases their coherence. We then turn to astrophysical batteries which can generate seed magnetic fields. Here the coherence scale can be large, but the field strength is generally very small. These seed fields need to be further amplified and maintained by a dynamo to explain observed magnetic fields in galaxies. Basic ideas behind both small and large-scale turbulent dynamos are outlined. The small-scale dynamo may help to understand the first magnetization of young galaxies, while the large-scale dynamo is important for the generation of fields with scales larger than the stirring scale, as observed in nearby disk galaxies. The current theoretical challenges that turbulent dynamos encounter and their possible resolution are discussed. Full article
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
Open AccessReview
Revisiting the Equipartition Assumption in Star-Forming Galaxies
Received: 7 March 2019 / Revised: 27 March 2019 / Accepted: 28 March 2019 / Published: 8 April 2019
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Abstract
Energy equipartition between cosmic rays and magnetic fields is often assumed to infer magnetic field properties from the synchrotron observations of star-forming galaxies. However, there is no compelling physical reason to expect the same. We aim to explore the validity of the energy [...] Read more.
Energy equipartition between cosmic rays and magnetic fields is often assumed to infer magnetic field properties from the synchrotron observations of star-forming galaxies. However, there is no compelling physical reason to expect the same. We aim to explore the validity of the energy equipartition assumption. After describing popular arguments in favour of the assumption, we first discuss observational results that support it at large scales and how certain observations show significant deviations from equipartition at scales smaller than ≈ 1 kpc , probably related to the propagation length of the cosmic rays. Then, we test the energy equipartition assumption using test-particle and magnetohydrodynamic (MHD) simulations. From the results of the simulations, we find that the energy equipartition assumption is not valid at scales smaller than the driving scale of the ISM turbulence (≈ 100 pc in spiral galaxies), which can be regarded as the lower limit for the scale beyond which equipartition is valid. We suggest that one must be aware of the dynamical scales in the system before assuming energy equipartition to extract magnetic field information from synchrotron observations. Finally, we present ideas for future observations and simulations to investigate in more detail under which conditions the equipartition assumption is valid or not. Full article
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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