Enhancement of magnetic fields arising from galactic encounters

Context. Galactic encounters are usually marked by a substantial increase in synchrotron emission of the interacting galaxies when compared with the typical emission from similar non-interacting galaxies. This increase is believed to be associated with an increase in the star formation rate and the turbulent magnetic fields resulting from the encounter, while the regular magnetic field is usually believed to decrease as a result of the encounter. Aims. We attempt to verify these expectations. Methods. We consider a simple, however rather realistic, mean-field galactic dynamo model where the effects of small-scale generation are represented by random injections of magnetic field resulting from star forming regions. We represent an encounter by the introduction of large-scale streaming velocities and by an increase in small-scale magnetic field injections. The latter describes the effect of an increase in the star formation rate caused by the encounter. Results. We demonstrate that large-scale streaming, with associated deviations in the rotation curve, can result in an enhancement of the anisotropic turbulent (ordered) magnetic field strength, mainly along the azimuthal direction. This leads to a significant temporary increase of the total magnetic energy during the encounter; the representation of an increase in star formation rate has an additional strong effect. In contrast to expectations, the large-scale (regular) magnetic field structure is not significantly destroyed by the encounter. It may be somewhat weakened for a relatively short period, and its direction after the encounter may be reversed. Conclusions. The encounter causes enhanced total and polarized emission without increase in the regular magnetic field strength. The increase in synchrotron emission caused by the large-scale streaming can be comparable to the effect of the increase in the star formation rate, depending on the choice of parameters. The effects of the encounter on the total magnetic field energy last only slightly longer than the duration of the encounter (ca. 1 Gyr). However, a long-lasting field reversal of the regular magnetic field may result.