The relation between magnetic and material arms in models for spiral galaxies

Abstract

Context. Observations of polarized radio emission show that large-scale (regular) magnetic fields in spiral galaxies are not axisymmetric, but generally stronger in interarm regions. In some nearby galaxies such as NGC 6946 they are organized in narrow magnetic arms situated between the material spiral arms. Aims. The phenomenon of magnetic arms and their relation to the optical spiral arms (the material arms) call for an explanation in the framework of galactic dynamo theory. Several possibilities have been suggested but are not completely satisfactory; here we attempt a consistent investigation. Methods. We use a 2D mean-field dynamo model in the no-z approximation and add injections of small-scale magnetic field, taken to result from supernova explosions, to represent the effects of dynamo action on smaller scales. This injection of small scale field is situated along the spiral arms, where star-formation mostly occurs. Results. A straightforward explanation of magnetic arms as a result of modulation of the dynamo mechanism by material arms struggles to produce pronounced magnetic arms, at least with realistic parameters, without introducing new effects such as a time lag between Coriolis force and α-effect. In contrast, by taking into account explicitly the small-scale magnetic field that is injected into the arms by the action of the star forming regions that are concentrated there, we can obtain dynamo models with magnetic structures of various forms that can be compared with magnetic arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered as coherent magnetic structures generated by large-scale dynamo action, and associated with spatially modulated small-scale magnetic fluctuations, caused by enhanced star formation rates within the material arms.

13 pages, 8 figures, accepted for publication to A&A

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• Non-linear galactic dynamos: A toolbox
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• Toward a hybrid dynamo model for the Milky Way
• Magnetic and gaseous spiral arms in M83
• Magnetic fields in primordial accretion disks
• Magnetic Spiral Arms in Galaxy Halos
• The formation of regular interarm magnetic fields in spiral galaxies
• Evolving Galactic Dynamos and Fits to the Reversing Rotation Measures in the Halo of NGC 4631
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• Simulations of galactic dynamos
• Non-linear Galactic Dynamos and the Magnetic Rädler Effect
• Magnetic fields near the peripheries of galactic discs
• The origin of the structure of large-scale magnetic fields in disc galaxies
• Magnetic fields in ring galaxies
• Hot gas and magnetic arms of NGC 6946: indications for reconnection heating?
• Probing magnetic fields with Square Kilometre array and its precursors
• NGC 5236's stars as tracers of arms and arm widths in spiral galaxies
• Towards convergence of turbulent dynamo amplification in cosmological simulations of galaxies