MODELS FOR THE MAGNETIC-FIELD OF M81

We study several mean field dynamo models in disk geometry in an attem pt to understand the origin of the nonaxisymmetric magnetic field pres ent in M81. There appear to be three (at least) relevant mechanisms, w hich are not mutually exclusive. Because field growth times are not ve ry short compared to galactic ages, a predominantly nonaxisymmetric se ed field may still give a significantly nonaxisymmetric field after ti mes of order 10(10) yr, even if the stable field configuration is axis ymmetric. The spiral structure may give a non-axisymmetric structure t o the disk turbulence, and thus to the turbulent coefficients appearin g in mean field dynamo theory. Third, M81 may have undergone a close e ncounter with a companion galaxy. A dynamical model of the interaction predicts strong, nonaxisymmetric, large-scale gas velocities in the d isk plane, and these can produce nonaxisymmetric fields. In the absenc e of the second of these effects, our models predict that nonaxisymmet ric fields will be present in the outer parts of the galaxy, together with significant axisymmetric contributions in the inner part. However , we do not find that any of these effects, taken individually, can pr oduce dominant nonaxisymmetric field structure. If they are simultaneo usly present, they can reinforce one another. Further, our calculation s are for a relatively thick disk (thickness to radius ratio of order 0.2), and a reduction to smaller, and plausibly more realistic, values will also favor nonaxisymmetric field generation.