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


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Magnetic arms in NGC 6946 generated by a turbulent dynamo

R. Rohde 1, R. Beck 2 and D. Elstner 1

1 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 1 June 1999 / Accepted 26 August 1999

Abstract

Dynamo models for the structure of the regular magnetic field in the galaxy NGC 6946 are confronted with recent observations of polarized radio emission at [FORMULA] cm. The observed behaviour of the concentration of regular fields between the gaseous spiral arms ("magnetic spiral arms") is achieved via a non-axisymmetric azimuthal distribution of only the correlation time [FORMULA] of interstellar turbulence, determined by the typical size of interstellar clouds or the lifetime of supernova remnants or superbubbles. There is no need for the turbulent gas velocity of gas clouds to vary between arm and interarm regions. Furthermore, a satisfactory agreement between model and observations of the radial distribution of the regular field strength requires that the galaxy rotates differentially at radii beyond a few 100 pc, as indicated by observations of the molecular gas. Our model also explains the strong concentration of regular fields within the inner 2 kpc as well as the observed arm/interarm variation of the field pitch angle.

Key words: Magnetohydrodynamics (MHD) – ISM: magnetic fields – galaxies: individual: – galaxies: ISM – galaxies: magnetic fields – galaxies: spiral

Send offprint requests to: R. Rohde (rrohde@aip.de)

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999

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