Magnetic fields in ring galaxies
  Many galaxies contain magnetic fields supported by galactic dynamo action.
However, nothing definitive is known about magnetic fields in ring galaxies.
Here we investigate large-scale magnetic fields in a previously unexplored
context, namely ring galaxies, and concentrate our efforts on the structures
that appear most promising for galactic dynamo action, i.e. outer star-forming
rings in visually unbarred galaxies. We use tested methods for modelling
$\alpha-\Omega$ galactic dynamos, taking into account the available
observational information concerning ionized interstellar matter in ring
galaxies. Our main result is that dynamo drivers in ring galaxies are strong
enough to excite large-scale magnetic fields in the ring galaxies studied. The
variety of dynamo driven magnetic configurations in ring galaxies obtained in
our modelling is much richer than that found in classical spiral galaxies. In
particular, various long-lived transients are possible. An especially
interesting case is that of NGC 4513 where the ring counter-rotates with
respect to the disc. Strong shear in the region between the disc and the ring
is associated with unusually strong dynamo drivers for the counter-rotators.
The effect of the strong drivers is found to be unexpectedly moderate. With
counter-rotation in the disc, a generic model shows that a steady mixed parity
magnetic configuration, unknown for classical spiral galaxies, may be excited,
although we do not specifically model NGC 4513. We deduce that ring galaxies
constitute a morphological class of galaxies in which identification of
large-scale magnetic fields from observations of polarized radio emission, as
well as dynamo modelling, may be possible. Such studies have the potential to
throw additional light on the physical nature of rings, their lifetimes and
evolution.
