Luke Chamandy, Kandaswamy Subramanian, Alice Quillen
Interfering two- and three-arm spiral patterns have previously been inferred to exist in many galaxies and also in numerical simulations, and invoked to explain important dynamical properties, such as lack of symmetry, kinks in spiral arms, and star formation in armlets. The non-axisymmetric galactic mean-field dynamo model of Chamandy et al. 2013a is generalized to allow for such multiple co-existing spiral patterns in the kinetic alpha effect, leading to the existence of magnetic spiral arms in the large-scale magnetic field with several new properties. The large-scale magnetic field produced by an evolving superposition of two- and three-arm patterns evolves with time along with the superposition. Magnetic arms can be stronger and more extended in radius and in azimuth when produced by two interfering patterns rather than by one pattern acting alone. Transient morphological features arise in the magnetic arms, including bifurcations, disconnected armlets, and temporal and spatial variation in arm strength and winding angle. The magnetic field is sometimes strongest in between the \alpha_k-arms, unlike in standard models with a single pattern, where it is strongest within the \alpha_k-arms. Moreover, some amount of m=1 azimuthal symmetry is found to be present in the magnetic field, which is generally not the case for forcing by single two- or three-arm patterns. Many of these results are reminiscent of observed features in the regular magnetic fields of nearby spiral galaxies, like NGC 6946 and IC 342, which have previously been inferred to have significant two- and three-arm spiral patterns.
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http://arxiv.org/abs/1308.0432
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