Magnetic Fields in Spiral Galaxies
(Sep 15, 2015)Abstract
Radio synchrotron emission is a powerful tool to study the strength and
structure of magnetic fields in galaxies. Unpolarized synchrotron emission
traces isotropic turbulent fields which are strongest in spiral arms and bars
(20-30G) and in central starburst regions (50-100G). Such fields are
dynamically important; they affect gas flows and drive gas inflows in central
regions.
Polarized emission traces ordered fields, which can be regular or anisotropic
turbulent, where the latter originates from isotropic turbulent fields by the
action of compression or shear. The strongest ordered fields (10-15G) are
generally found in interarm regions. In galaxies with strong density waves,
ordered fields are also observed at the inner edges of spiral arms. Ordered
fields with spiral patterns exist in grand-design, barred and flocculent
galaxies, and in central regions. Ordered fields in interacting galaxies have
asymmetric distributions and are a tracer of past interactions between galaxies
or with the intergalactic medium.
Faraday rotation measures of the diffuse polarized radio emission from galaxy
disks reveal large-scale spiral patterns that can be described by the
superposition of azimuthal modes; these are signatures of regular fields
generated by mean-field dynamos. "Magnetic arms" between gaseous spiral arms
may also be products of dynamo action, but need a stable spiral pattern to
develop. Helically twisted field loops winding around spiral arms were found in
two galaxies so far. Large-scale field reversals, like the one found in the
Milky Way, could not yet be detected in external galaxies.
In radio halos around edge-on galaxies, ordered magnetic fields with X-shaped
patterns are observed. Halo fields of even symmetry probably dominate.
The origin and evolution of cosmic magnetic fields will be studied with
forthcoming radio telescopes like the Square Kilometre Array.