CHANG-ES XXI. Transport processes and the X-shaped magnetic field of NGC
  4217: off-center superbubble structure
  In order to gain a better understanding of the influence of cosmic rays (CRs)
and magnetic fields in the disk-halo interface of edge-on spiral galaxies, we
investigate the radio continuum halo, the magnetic field, and the transport
processes of the CRs of the edge-on spiral galaxy NGC 4217 using CHANG-ES radio
data at two frequencies, 6 GHz (C-band) and 1.5 GHz (L-band), and supplemental
LOFAR data at 150 MHz and X-ray Chandra data.
  NGC 4217 shows a large-scale X-shaped magnetic field structure, covering a
major part of the galaxy with a mean total magnetic field strength in the disk
of 9 micro Gauss (via equipartition). Using rotation measure synthesis
(RM-synthesis) at C-band, we found that the direction of the disk magnetic
field is pointing inward. A helical outflow structure is furthermore present in
the northwestern part of the galaxy, which is extended nearly 7 kpc into the
halo. More polarized emission is observed on the approaching side of the
galaxy. With a simplified galaxy disk model, we are able to explain that
finding and predict that roughly 75% of edge-on spiral galaxies will show
higher polarized intensity on the approaching side. Many loop and shell
structures are found throughout the galaxy in total intensity at C-band. A
superbubble-like structure is prominent in total and polarized intensity, as
well as in Halpha and optical dust filaments, being a possible result of
concentrated star formation in the disk. The flux density contribution of the
disk in comparison to the halo decreases toward lower frequencies. Total
intensity profiles at the three radio frequencies were fit with two-component
exponential functions. The frequency dependence of the resulting scale heights
between C-band and L-band suggests advection to be the main CR transport
process. The 1D CR transport modeling (SPINNAKER) shows that advection appears
to be more important than diffusion.
