1995A&A...301...11V

Dynamical processes, such as tidal interactions, can have a profound influence on the magnetic field structure in galactic disks. Very few galaxies lived quiescent existences. Traces of past dynamical activities of a spiral galaxy may be found in the shape of its neutral hydrogen (H I) gas distribution, in the form of morphological deformations (MD). Thus fossil H I gas may point a finger towards a passing neighbor galaxy. Galactic dynamo states of spiral galaxies can be found in the shape of the global magnetic field distribution, more generally in its azimuthal mode (m_azim). Global magnetic modes can be axisymmetric or bisymmetric. Here, MD examples are obtained for NGC 891, NGC 4631, and the Large Magellanic Cloud (LMC), for which m_azim has recently been suggested. Also, I find many observational evidence to link statistically galactic dynamos and galactic dynamics, via their parameters m_azim and MD. A more complex H I morphological shape (MD value) goes along with a more complex magnetic field shape (m_azim value), and vice-versa, e.g. Table 2 and Fig. 5. This statistical link points to a necessary common cause for creating H I fossils and global magnetic shapes, ie most likely a deformed gravitational potential around a spiral galaxy. The deformation of the galactic gravitational potential may be caused by a tidal encounter with a nearby companion galaxy, or by a long rotating bar straddling the galactic nucleus. Such a deformed gravitational potential would require new terms in the general dynamo equations employed to date for galactic magnetism. The time-evolution of the deformed gravitational potential could be the main trigger to change the m_azim from axisymmetric (0), to bisymmetric (1), and vice-versa.