Radio broadband visualization of global three-dimensional
  magneto-hydrodynamical simulations of spiral galaxies II. Faraday
  Depolarization from 100MHz to 10GHz
  Observational study of galactic magnetic fields is limited by projected
observables. Comparison with numerical simulations is helpful to understand the
real structures, and observational visualization of numerical data is an
important task. Machida et al. (2018) have reported Faraday depth maps obtained
from numerical simulations. They showed that the relation between azimuthal
angle and Faraday depth depends on the inclination angle. In this paper, we
investigate 100MHz to 10GHz radio synchrotron emission from spiral galaxies,
using the data of global three-dimensional magneto-hydrodynamic simulations. We
model internal and external Faraday depolarization at small scales and assume a
frequency independent depolarization. It is found that the internal and
external Faraday depolarization becomes comparable inside the disk and the
dispersion of Faraday depth becomes about 4rad/m^{2} for face-on view and
40rad/m2 for edge-on view, respectively. The internal depolarization becomes
ineffective in the halo. Because of the magnetic turbulence inside the disk,
frequency independent depolarization works well and the polarization degree
becomes 0.3 at high frequency. When the observed frequency is in the 100 MHz
band, polarized intensity vanishes in the disk, while that from the halo can be
observed. Because the remaining component of polarized intensity is weak in the
halo and the polarization degree is about a few %, it may be difficult to
observe that component. These results indicate that the structures of global
magnetic fields in spiral galaxies could be elucidated, if broadband
polarimetry such as that with the Square Kilometre Array is achieved.
