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21 pages, 7017 KiB

Open AccessFeature PaperArticle

Magnetic Fields in Molecular Clouds—Observation and Interpretation

by Hua-Bai Li

Galaxies 2021, 9(2), 41; https://doi.org/10.3390/galaxies9020041 - 8 Jun 2021

Cited by 20 | Viewed by 10952

Abstract

The Zeeman effect and dust grain alignment are two major methods for probing magnetic fields (B-fields) in molecular clouds, largely motivated by the study of star formation, as the B-field may regulate gravitational contraction and channel turbulence velocity. This review summarizes our observations [...] Read more.

The Zeeman effect and dust grain alignment are two major methods for probing magnetic fields (B-fields) in molecular clouds, largely motivated by the study of star formation, as the B-field may regulate gravitational contraction and channel turbulence velocity. This review summarizes our observations of B-fields over the past decade, along with our interpretation. Galactic B-fields anchor molecular clouds down to cloud cores with scales around 0.1 pc and densities of 10^4–5 H₂/cc. Within the cores, turbulence can be slightly super-Alfvénic, while the bulk volumes of parental clouds are sub-Alfvénic. The consequences of these largely ordered cloud B-fields on fragmentation and star formation are observed. The above paradigm is very different from the generally accepted theory during the first decade of the century, when cloud turbulence was assumed to be highly super-Alfvénic. Thus, turbulence anisotropy and turbulence-induced ambipolar diffusion are also revisited. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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25 pages, 1861 KiB

Open AccessEditor’s ChoiceArticle

Parameters of the Supernova-Driven Interstellar Turbulence

by Luke Chamandy and Anvar Shukurov

Galaxies 2020, 8(3), 56; https://doi.org/10.3390/galaxies8030056 - 31 Jul 2020

Cited by 12 | Viewed by 3212

Abstract

Galactic dynamo models take as input certain parameters of the interstellar turbulence, most essentially the correlation time

τ

, root-mean-square turbulent speed u, and correlation scale l. However, these quantities are difficult, or, in the case of

τ

, impossible, to [...] Read more.

Galactic dynamo models take as input certain parameters of the interstellar turbulence, most essentially the correlation time

τ

, root-mean-square turbulent speed u, and correlation scale l. However, these quantities are difficult, or, in the case of

τ

, impossible, to directly observe, and theorists have mostly relied on order of magnitude estimates. Here we present an analytic model to derive these quantities in terms of a small set of more accessible parameters. In our model, turbulence is assumed to be driven concurrently by isolated supernovae (SNe) and superbubbles (SBs), but clustering of SNe to form SBs can be turned off if desired, which reduces the number of model parameters by about half. In general, we find that isolated SNe and SBs can inject comparable amounts of turbulent energy into the interstellar medium, but SBs do so less efficiently. This results in rather low overall conversion rates of SN energy into turbulent energy of ∼1–3%. The results obtained for l, u and

τ

for model parameter values representative of the Solar neighbourhood are consistent with those determined from direct numerical simulations. Our analytic model can be combined with existing dynamo models to predict more directly the magnetic field properties for nearby galaxies or for statistical populations of galaxies in cosmological models. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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37 pages, 555 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Magnetism Science with the Square Kilometre Array

by George Heald, Sui Ann Mao, Valentina Vacca, Takuya Akahori, Ancor Damas-Segovia, B. M. Gaensler, Matthias Hoeft, Ivan Agudo, Aritra Basu, Rainer Beck, Mark Birkinshaw, Annalisa Bonafede, Tyler L. Bourke, Andrea Bracco, Ettore Carretti, Luigina Feretti, J. M. Girart, Federica Govoni, James A. Green, JinLin Han, Marijke Haverkorn, Cathy Horellou, Melanie Johnston-Hollitt, Roland Kothes, Tom Landecker, Błażej Nikiel-Wroczyński, Shane P. O’Sullivan, Marco Padovani, Frédérick Poidevin, Luke Pratley, Marco Regis, Christopher John Riseley, Tim Robishaw, Lawrence Rudnick, Charlotte Sobey, Jeroen M. Stil, Xiaohui Sun, Sharanya Sur, A. Russ Taylor, Alec Thomson, Cameron L. Van Eck, Franco Vazza, Jennifer L. West and the SKA Magnetism Science Working Group Show full author list Hide full author list

Galaxies 2020, 8(3), 53; https://doi.org/10.3390/galaxies8030053 - 6 Jul 2020

Cited by 64 | Viewed by 6195

Abstract

The Square Kilometre Array (SKA) will answer fundamental questions about the origin, evolution, properties, and influence of magnetic fields throughout the Universe. Magnetic fields can illuminate and influence phenomena as diverse as star formation, galactic dynamics, fast radio bursts, active galactic nuclei, large-scale [...] Read more.

The Square Kilometre Array (SKA) will answer fundamental questions about the origin, evolution, properties, and influence of magnetic fields throughout the Universe. Magnetic fields can illuminate and influence phenomena as diverse as star formation, galactic dynamics, fast radio bursts, active galactic nuclei, large-scale structure, and dark matter annihilation. Preparations for the SKA are swiftly continuing worldwide, and the community is making tremendous observational progress in the field of cosmic magnetism using data from a powerful international suite of SKA pathfinder and precursor telescopes. In this contribution, we revisit community plans for magnetism research using the SKA, in light of these recent rapid developments. We focus in particular on the impact that new radio telescope instrumentation is generating, thus advancing our understanding of key SKA magnetism science areas, as well as the new techniques that are required for processing and interpreting the data. We discuss these recent developments in the context of the ultimate scientific goals for the SKA era. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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14 pages, 606 KiB

Open AccessArticle

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

by Prasanta Bera, Anvar Shukurov and Kandaswamy Subramanian

Galaxies 2019, 7(4), 91; https://doi.org/10.3390/galaxies7040091 - 29 Nov 2019

Cited by 3 | Viewed by 2658

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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28 pages, 5305 KiB

Open AccessArticle

An In-Depth Investigation of Faraday Depth Spectrum Using Synthetic Observations of Turbulent MHD Simulations

by Aritra Basu, Andrew Fletcher, Sui Ann Mao, Blakesley Burkhart, Rainer Beck and Dominic Schnitzeler

Galaxies 2019, 7(4), 89; https://doi.org/10.3390/galaxies7040089 - 23 Nov 2019

Cited by 16 | Viewed by 3727

Abstract

In this paper, we present a detailed analysis of the Faraday depth (FD) spectrum and its clean components obtained through the application of the commonly used technique of Faraday rotation measure synthesis to analyze spectro-polarimetric data. To directly compare the Faraday depth spectrum [...] Read more.

In this paper, we present a detailed analysis of the Faraday depth (FD) spectrum and its clean components obtained through the application of the commonly used technique of Faraday rotation measure synthesis to analyze spectro-polarimetric data. To directly compare the Faraday depth spectrum with physical properties of a magneto-ionic medium, we generated synthetic broad-bandwidth spectro-polarimetric observations from magnetohydrodynamic (MHD) simulations of a transonic, isothermal, compressible turbulent medium. We find that correlated magnetic field structures give rise to a combination of spiky, localized peaks at certain FD values, and broad structures in the FD spectrum. Although most of these spiky FD structures appear narrow, giving an impression of a Faraday thin medium, we show that they arise from strong synchrotron emissivity at that FD. Strong emissivity at a FD can arise because of both strong spatially local polarized synchrotron emissivity at a FD or accumulation of weaker emissions along the distance through a medium that have Faraday depths within half the width of the rotation measure spread function. Such a complex Faraday depth spectrum is a natural consequence of MHD turbulence when the lines of sight pass through a few turbulent cells. This therefore complicates the convention of attributing narrow FD peaks to the presence of a Faraday-rotating medium along the line of sight. Our work shows that it is difficult to extract the FD along a line of sight from the Faraday depth spectrum using standard methods for a turbulent medium in which synchrotron emission and Faraday rotation occur simultaneously. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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12 pages, 7552 KiB

Open AccessArticle

Magnetic Fields and Halos in Spiral Galaxies

by Marita Krause

Galaxies 2019, 7(2), 54; https://doi.org/10.3390/galaxies7020054 - 4 May 2019

Cited by 11 | Viewed by 5464

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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22 pages, 758 KiB

Open AccessArticle

Solar and Galactic Magnetic Halo Structure: Force-Free Dynamos?

by Richard Henriksen

Galaxies 2019, 7(2), 53; https://doi.org/10.3390/galaxies7020053 - 3 May 2019

Cited by 3 | Viewed by 3276

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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18 pages, 4469 KiB

Open AccessArticle

Faraday Rotation of Extended Emission as a Probe of the Large-Scale Galactic Magnetic Field

by Anna Ordog, Rebecca A. Booth, Cameron L. Van Eck, Jo-Anne C. Brown and Thomas L. Landecker

Galaxies 2019, 7(2), 43; https://doi.org/10.3390/galaxies7020043 - 27 Mar 2019

Cited by 12 | Viewed by 3754

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^{\circ} < ℓ < 193^{\circ}, - 3^{\circ} < b < 5^{\circ}

, 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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10 pages, 267 KiB

Open AccessArticle

Magnetic Fields Around Galactic Discs

by David Moss and Dmitry Sokoloff

Galaxies 2019, 7(1), 36; https://doi.org/10.3390/galaxies7010036 - 7 Mar 2019

Cited by 6 | Viewed by 2977

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

45 pages, 1834 KiB

Open AccessReview

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

by Rainer Beck, Luke Chamandy, Ed Elson and Eric G. Blackman

Galaxies 2020, 8(1), 4; https://doi.org/10.3390/galaxies8010004 - 21 Dec 2019

Cited by 74 | Viewed by 5068

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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25 pages, 4533 KiB

Open AccessEditor’s ChoiceReview

Practical Modeling of Large-Scale Galactic Magnetic Fields: Status and Prospects

by Tess R. Jaffe

Galaxies 2019, 7(2), 52; https://doi.org/10.3390/galaxies7020052 - 28 Apr 2019

Cited by 41 | Viewed by 5641

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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19 pages, 401 KiB

Open AccessEditor’s ChoiceReview

From Primordial Seed Magnetic Fields to the Galactic Dynamo

by Kandaswamy Subramanian

Galaxies 2019, 7(2), 47; https://doi.org/10.3390/galaxies7020047 - 14 Apr 2019

Cited by 49 | Viewed by 5119

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)

20 pages, 3037 KiB

Open AccessEditor’s ChoiceReview

Revisiting the Equipartition Assumption in Star-Forming Galaxies

by Amit Seta and Rainer Beck

Galaxies 2019, 7(2), 45; https://doi.org/10.3390/galaxies7020045 - 8 Apr 2019

Cited by 37 | Viewed by 4880

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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