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Astron. Astrophys. 350, 423-433 (1999)

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1. Introduction

The phenomenon of interarm magnetic fields ("magnetic arms"), first discovered in NGC 6946, has been discussed from several points of view. Beck & Hoernes (1996) mentioned four possible explanations - density waves, instabilities, magnetic reconnection, dynamo theory. Until now some progress was achieved. Fan & Lou (1996) and Lou & Fan (1998) discussed the possibility that interarm magnetic fields are due to slow MHD density waves. However, such waves may exist only in the rigidly rotating part of a galaxy. Other attempts involved dynamo theory in the framework of a mean-field approach. Recent numerical simulations of a 3D nonlinear galactic dynamo concerned with interarm magnetic fields were investigated by Rohde & Elstner (1998) where the excitation of magnetic fields between the optical spiral arms was due to an enhanced turbulence intensity (and density) within the arms. Schreiber & Schmitt (1999) discussed a 3D linear galactic dynamo also achieving enhanced interarm magnetic fields since the [FORMULA] effect, the turbulent diffusivity and the large-scale velocity are all modulated by a stationary density wave. The occurrence of interarm magnetic fields in terms of dynamo numbers that describe the interplay between field inducing effects (differential rotation, [FORMULA]-effect and diffusivity) was discussed by Shukurov (1998) and Shukurov & Sokoloff (1998) for models with density and turbulence intensity both enhanced in the optical arms. With a 2D approximation Moss (1998) investigated nonlinear dynamos with spiral arms that are introduced via large-scale non-axisymmetric gas velocities, non-axisymmetric turbulent diffusivity and non-axisymmetric [FORMULA]-effect, respectively. All models show magnetic arms in interarm regions under the assumption of enhanced turbulent velocity in the optical arms. However, no observational hints were found yet in support of this assumption (see Sect. 4.2).

In the present investigation we try to get further insight into the magnetic field structure of NGC 6946 assuming that the field is generated by a nonlinear turbulent dynamo as introduced by Rohde & Elstner (1998). The excited magnetic field is discussed with respect to several properties of NGC 6946 as the distribution of gas density and turbulence intensity, rotation curve, corotation radius and spiral arm shape.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999