Abstract
Tidal interactions between neighbouring galaxies are expected to induce significant nonaxisymmetric velocities in their disks. It has been suggested that these velocities play an important role in the generation of bisymmetric magnetic fields observed in interacting galaxies. We investigate the effect of a nonaxisymmetric radial outflow on a three-dimensional linear mean field dynamo. We find that the usually dominant axisymmetric quadrupole is effectively damped by the outflow. For sufficiently high velocities a bisymmetric magnetic mode is then preferentially excited. The resulting field has a spiral-arm structure extending well into the differentially rotating outskirts of the disk. The influence of velocity-induced mode-coupling effects on bisymmetric field generation is found to be negligible. While being highly idealized, the model seems to give reasonable representations of the large scale fields of the interacting galaxies M 81 and M 51.
Highlights
Radio observations indicate that in some galaxies, notably M 81 and M 51, and M 33 and NGC 2276, the globalscale magnetic field has a nonaxisymmetric component
It has been suggested that these velocities play an important role in the generation of bisymmetric magnetic fields observed in interacting galaxies
We investigate the effect of a nonaxisymmetric radial outflow on a three-dimensional linear mean field dynamo
Summary
Radio observations indicate that in some galaxies, notably M 81 and M 51, and M 33 and NGC 2276, the globalscale magnetic field has a nonaxisymmetric component. While application of standard mean field dynamo theory predicts dominant axisymmetric fields (Beck et al 1996), several suggestions have been made to overcome the destructive effect that differential rotation of the galactic disk has on bisymmetric field configurations (Radler 1986). These include azimutal modulation of the α-effect (Moss et al 1993; Rohde et al 1999; Schreiber & Schmitt 2000), long
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