Simulations of Galactic Dynamos

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 Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies.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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“…Previous reviews of galactic dynamos include Refs. [19,[39][40][41][42][43][44] as well as Refs. [45,46].…”

Section: Galactic Dynamo Theory and Simulationsmentioning

confidence: 99%

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

et al. 2019

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“…It can be envisaged, however, that a strong density wave (even axisymmetric) comprises fluid motions that are propitious for kinematic dynamo action. Its significant radial compression, allied with baroclinicity, may generate a vortical flow in the x − z plane, an important ingredient for small-scale dynamo action (Brandenburg 2015). But this poloidal circulation when combined with the orbital shear provides helicity, important for large-scale dynamo action.…”

Section: The Spiral Wave Dynamomentioning

confidence: 99%

Magnetorotational instability and dynamo action in gravito-turbulent astrophysical discs

Riols

Latter

2017

Monthly Notices of the Royal Astronomical Society

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Though usually treated in isolation, the magnetorotational and gravitational instabilities (MRI and GI) may coincide at certain radii and evolutionary stages of protoplanetary discs and active galactic nuclei. Their mutual interactions could profoundly influence several important processes, such as accretion variability and outbursts, fragmentation and disc truncation, or large-scale magnetic field production. Direct numerical simulations of both instabilities are computationally challenging and remain relatively unexplored. In this paper, we aim to redress this neglect via a set of 3D vertically stratified shearing-box simulations, combining self-gravity and magnetic fields. We show that gravito-turbulence greatly weakens the zero-net-flux MRI. In the limit of efficient cooling (and thus enhanced GI), the MRI is completely suppressed, and yet strong magnetic fields are sustained by the gravitoturbulence. This turbulent 'spiral wave' dynamo may have widespread application, especially in galactic discs. Finally, we present preliminary work showing that a strong net-vertical-flux revives the MRI and supports a magnetically dominated state, in which the GI is secondary.

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“…which is essentially a Reynolds number ( of the sub-scale turbulence) and is essentially the dynamo number used elsewhere Brandenburg (2014). The velocity components {u, v, w} are now in Units ofηδ, that is v = (ηδ) (u, v, w).…”

Section: Kinematic Dynamo Theorymentioning

confidence: 99%