Magnetic spiral arms in galaxy haloes

Henriksen, R. N.

doi:10.1093/mnras/stx1169

Cited by 20 publications

(33 citation statements)

References 16 publications

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“…Work on the former topic can be found in a series of papers beginning with Moss et al [208] and Beck et al [209]. The latter topic has been studied observationally [8] for M 51 and theoretically using analytical mean-field models that assume self-similarity [210].…”

Section: Non-axisymmetric Large-scale Fieldsmentioning

confidence: 99%

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

et al. 2019

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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 Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies.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. Further, 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.

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Section: Non-axisymmetric Large-scale Fieldsmentioning

confidence: 99%

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

et al. 2019

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“…We refer to the classical mean-field dynamo equations (Moffatt 1978) in the form for the magnetic vector potential (Henriksen 2017b)…”

Section: Scale Invariant Evolving Magnetic Dynamo Spiral Fieldsmentioning

confidence: 99%

“…Here ∆ is the inverse of the definition used in Henriksen (2017b) in order to treat it as small when we wish to neglect diffusion. It might be a function of ζ at this stage.…”

Section: Scale Invariant Evolving Magnetic Dynamo Spiral Fieldsmentioning

confidence: 99%

Evolving galactic dynamos and fits to the reversing rotation measures in the halo of NGC 4631

Woodfinden

Henriksen

Irwin

et al. 2019

Monthly Notices of the Royal Astronomical Society

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Rotation measure (RM) synthesis maps of NGC 4631 show remarkable sign reversals with distance from the minor axis in the northern halo of the galaxy on kpc scales. To explain this new phenomenon, we solve the dynamo equations under the assumption of scale invariance and search for rotating logarithmic spiral solutions. Solutions for velocity fields representing accretion onto the disk, outflow from the disk, and rotationonly in the disk are found that produce RM with reversing signs viewed edge-on. Model RM maps are created for a variety of input parameters using a Faraday screen and are scaled to the same amplitude as the observational maps. Residual images are then made and compared to find a best fit. Solutions for rotation-only, i.e. relative to a pattern uniform rotation, did in general, not fit the observations of NGC 4631 well. However, outflow models did provide a reasonable fit to the magnetic field. The best results for the specific region that was modelled in the northern halo are found with accretion. Since there is abundant evidence for both winds and accretion in NGC 4631, this modelling technique has the potential to distinguish between the dominant flows in galaxies.

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“…Recently there has been some progress both in the measurement of galactic halo magnetic fields ( [1,2]), and in the analytic theory of corresponding mean field galactic dynamos. A summary and references to earlier work can be found in [3], while recent analytic developments, using the assumption of scale invariance to treat the 3D problem, can be found in a series of papers [4][5][6]. These papers mostly start from the classical mean field theory [7], and study both axially symmetric dynamo fields and spirally bi-symmetric fields.…”

Section: Introductionmentioning

confidence: 99%

“…In Section 2 below we give our formulation of the non linear, axially symmetric, force-free, magnetic field, including a scale invariant version (see e.g., [5]). In the latter case a detailed comparison with ( [22]) is given.…”

Section: Introductionmentioning

confidence: 99%

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

Henriksen

2019

Galaxies

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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.

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Magnetic spiral arms in galaxy haloes

Abstract: We seek the conditions for a steady mean field galactic dynamo. The parameter set is reduced to those appearing in the α 2 and α/ω dynamo, namely velocity amplitudes, and the ratio of sub-scale helicity to diffusivity. The pa-

Cited by 20 publications

References 16 publications

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

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

Evolving galactic dynamos and fits to the reversing rotation measures in the halo of NGC 4631

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

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