Synthesizing Observations and Theory to Understand Galactic Magnetic
  Fields: Progress and Challenges
  Constraining dynamo theories of magnetic field origin by observation is
indispensable but challenging, in part because the basic quantities measured by
observers and predicted by modelers are different. We clarify these differences
and sketch out ways to bridge the divide. Based on archival and previously
unpublished data, we then compile various important properties of galactic
magnetic fields for nearby spiral galaxies. We consistently compute strengths
of total, ordered, and regular fields, pitch angles of ordered and regular
fields, and we summarize the present knowledge on azimuthal modes, field
parities, and the properties of non-axisymmetric spiral features called
magnetic arms. We review related aspects of dynamo theory, with a focus on
mean-field models and their predictions for large-scale magnetic fields in
galactic discs and halos. Further, we measure the velocity dispersion of HI gas
in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6496, since
spiral modulation of the root-mean-square turbulent speed has been proposed as
a driver of non-axisymmetry in large-scale dynamos. We find no evidence for
such a modulation and place upper limits on its strength, helping to narrow
down the list of mechanisms to explain magnetic arms. Successes and remaining
challenges of dynamo models with respect to explaining observations are briefly
summarized, and possible strategies are suggested. With new instruments like
the Square Kilometre Array (SKA), large data sets of magnetic and non-magnetic
properties from thousands of galaxies will become available, to be compared
with theory.
