Modeling of magnetic field structures in irregular galaxies of Magellanic type

The recently observed strong-regular magnetic fields in the dwarf irregular (IBm) galaxy NGC 4449 are not predicted by classical turbulent dynamo theo ry. The very slow and al most unorganized rotation of the galaxy appears to contradict the observed large-scale magnetic fields manifested in a fan li ke 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 evolut ion of magnetic fields. We invoke a model of an irregular galaxy by numeric ally solving the kinematic dynamo equation and adopting a gas velocity fiel d obtained from N-body simulations. We report that the puzzling magnetic fr agments 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 act ion, is needed to maintain the high magnetic energy in the presence of a tu rbulent diffusion. To model the observed magnetic fields in the central par t of NGC 4449 we incorporate high stellar activity (observed within the mai n body of the galaxy) as a source of random magnetic fields. Additionally, we include a spherical outflow to reproduce the radially oriented " fans" o f magnetic field that are observed. The inclusion of this outflow, however, does not lead to the observed radio polarization properties and the magnet ic pitch angle distribution in the central part of the galaxy, so a more re alistic model of an outflow with regard to the star formation distribution, is needed.