Magnetic Fields Around Galactic Discs

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 Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies.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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“…28,202]. For a more extensive review of magnetic fields around (as opposed to in) galactic discs, see Moss and Sokoloff [45].…”

Section: Dynamo Modelsmentioning

confidence: 99%

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

et al. 2019

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“…Since the magnetic fields have zero divergences ▿. B = 0 (the solenoidality condition), if B z = 0, for the axisymmetric spiral field (azimuthal mode m = 0, ASS), one can obtain ∂(ρB ρ )/∂ρ = 0, and then B ρ ∝ 1/ρ, thus giving a singularity on the axis (Moss & Sokoloff 2019). The singularity problem at the origin can be avoided by considering the components of the regular magnetic field for the ASS model as follows (Ruiz-Granados et al 2012):…”

Section: Axisymmetric Modelmentioning

confidence: 99%

Influence of Magnetic Fields on the Gas Rotation in the Galaxy NGC 6946

Khademi

Nasiri

Tabatabaei

2023

ApJ

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Magnetic fields can play an important role in the energy balance and formation of gas structures in galaxies. However, their dynamical effect on the rotation curve of galaxies is immensely unexplored. We investigate the dynamical effect of the known magnetic arms of NGC 6946 on its circular gas rotation traced in H i, considering two dark-matter mass-density models, ISO, and the universal NFW profile. We used a three-dimensional model for the magnetic field structure to fit the modeled rotation curve to the observed data via a χ 2 minimization method. The shape of the H i gas rotation curve is reproduced better including the effect of the magnetic field, especially in the outer part, where the dynamical effect of the magnetic field could become important. The typical amplitude of the regular magnetic field contribution in the rotation curve is about 6–14 km s−1 in the outer gaseous disk of the galaxy NGC 6946. The contribution ratio of the regular magnetic field to the observed circular velocity and to dark matter increases with the galactocentric radius. Its ratio to the observed rotational velocity is about 5% and, to dark matter, is about 10% in the outer regions of the galaxy NGC 6946. Therefore, the large-scale magnetic fields cannot be completely ignored in the large-scale dynamics of spiral galaxies, especially in the outer parts of galaxies.

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“…Nixon et al (2018) propose an initial poloidal field evolving by shear effects between the warm interstellar medium and the halo. However, the mechanism forming the observed X-shape is not considered to be well understood yet (Moss & Sokoloff 2019).…”

Section: Fields Outside the Galactic Diskmentioning

confidence: 99%

Exploring the Magnetic Field Geometry in NGC 891 with SOFIA/HAWC+

Kim

Jones

Dowell

2023

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Stratospheric Observatory For Infrared Astronomy/High-resolution Airborne Wideband Camera-plus 154 μm Far-Infrared polarimetry observations of the well-studied edge-on galaxy NGC 891 are analyzed and compared to simple disk models with ordered (planar) and turbulent magnetic fields. The overall low magnitude and the narrow dispersion of fractional polarization observed in the disk require significant turbulence and a large number of turbulent decorrelation cells along the line of sight through the plane. Higher surface brightness regions along the major axis to either side of the nucleus show a further reduction in polarization and are consistent with a view tangent to a spiral feature in our disk models. The nucleus also has a similar low polarization, and this is inconsistent with our model spiral galaxy where the ordered magnetic field component would be nearly perpendicular to the line of sight through the nucleus on an edge-on view. A model with a barred spiral morphology with a magnetic field geometry derived from radio synchrotron observations of face-on barred spirals fits the data much better. There is clear evidence for a vertical field extending into the halo from one location in the disk coincident with a polarization null point seen in near-infrared polarimetry, probably due to a blowout caused by star formation. Although our observations were capable of detecting a vertical magnetic field geometry elsewhere in the halo, no clear signature was found. A reduced polarization due to a mix of planar and vertical fields in the dusty regions of the halo best explains our observations, but unusually significant turbulence cannot be ruled out.

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Magnetic Fields Around Galactic Discs

Cited by 7 publications

References 46 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

Influence of Magnetic Fields on the Gas Rotation in the Galaxy NGC 6946

Exploring the Magnetic Field Geometry in NGC 891 with SOFIA/HAWC+

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