Regular magnetic fields in the dwarf irregular galaxy NGC 4449

We present a high-resolution VLA study of the total power and polarized rad io continuum emission at 8.46 and 4.86 GHz of the irregular galaxy NGC 4449 , known for its weak rotation and non-systematic gas motions. We found stro ng galaxy-scale regular magnetic fields, which is surprising because of a l ack of ordered rotation required for the dynamo action. The strength of the regular field reaches 8 mu G and that of the total field 14 mu G, comparab le to that of the total magnetic field strength in radio-bright spirals. Th e magnetic vectors in NGC 4449 form radial "fans" in the central region and fragments of a spiral pattern in the galaxy's outskirts. These structures are associated with large regions of systematic Faraday rotation, implying genuine galaxy-scale magnetic fields rather than random ones compressed and stretched by gas flows. The observed pattern of polarization B-vectors is similar to dynamotype fields in normal spirals. Nonstandard, fast dynamo co ncepts are required to explain the observed field strengths, though it is u nknown what kind of magnetic field geometry can be produced in slowly and c haotically rotating objects. The so far neglected role of magnetic fields f or the dynamics and star formation in dwarf irregulars also needs to be rev ised.