Properties of the Magneto-ionic Medium in the Halo of M51 revealed by
  Wide-band Polarimetry
  We present a study of the magneto-ionic medium in the Whirlpool galaxy (M51)
using new wide-band multi-configuration polarization data at L band (1-2 GHz)
obtained at the Karl G. Jansky Very Large Array. By fitting the observed
diffuse complex polarization $Q$+$iU$ as a function of wavelength directly to
various depolarization models, we find that polarized emission from M51 at 1-2
GHz originates from the top of the synchrotron disk and then experiences
Faraday rotation in the near-side thermal halo of the galaxy. Thus, the scale
height of the thermal gas must exceed that of the synchrotron emitting gas at L
band. The observed Faraday depth distribution at L band is consistent with a
halo field that comprises of a plane-parallel bisymmetric component and a
vertical component which produces a Faraday rotation of $\sim$ $-$9 rad
m$^{-2}$. The derived rotation measure structure functions indicate a
characteristic scale of rotation measure fluctuations of less than 560 pc in
the disk and approximately 1 kpc in the halo. The outer scale of turbulence of
1 kpc found in the halo of M51 is consistent with superbubbles and the Parker
instability being the main energy injection mechanisms in galactic halos.
