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Astron. Astrophys. 359, 29-40 (2000)


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Modeling of magnetic field structures in irregular galaxies of Magellanic type

K. Otmianowska-Mazur 1, K.T. Chyy 1, M. Soida 1 and S. von Linden 2

1 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
2 Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany

Received 18 June 1999 / Accepted 26 April 2000

Abstract

The recently observed strong-regular magnetic fields in the dwarf irregular (IBm) galaxy NGC 4449 are not predicted by classical turbulent dynamo theory. The very slow and almost unorganized rotation of the galaxy appears to contradict the observed large-scale magnetic fields manifested in a fan-like structure emanating from vigorous star formation in the central part of the galaxy and a spiral shell-like pattern on its periphery. In this paper, we investigate the influence of different physical processes on the evolution of magnetic fields. We invoke a model of an irregular galaxy by numerically solving the kinematic dynamo equation and adopting a gas velocity field obtained from N-body simulations. We report that the puzzling magnetic fragments of a polarized shell containing spiral magnetic fields in NGC 4449 can originate from shear motions of the interstellar gas associated with a bar perturbation. A significant role, recently postulated "fast" dynamo action, is needed to maintain the high magnetic energy in the presence of a turbulent diffusion. To model the observed magnetic fields in the central part of NGC 4449 we incorporate high stellar activity (observed within the main body of the galaxy) as a source of random magnetic fields. Additionally, we include a spherical outflow to reproduce the radially oriented "fans" of magnetic field that are observed. The inclusion of this outflow, however, does not lead to the observed radio polarization properties and the magnetic pitch angle distribution in the central part of the galaxy, so a more realistic model of an outflow with regard to the star formation distribution, is needed.

Key words: Magnetohydrodynamics (MHD) – galaxies: individual: NGC 4449 – galaxies: magnetic fields

Send offprint requests to: K. Otmianowska-Mazur

Correspondence to: otmian@oa.uj.edu.pl

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000

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