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 10 GHz radio synchrotron emission from spiral galaxies, using the data of global three-dimensional magnetohydrodynamic 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 disc and the dispersion of Faraday depth becomes about 4 radm-2 for face-on view and 40 radm-2 for edge-on view, respectively. The internal depolarization becomes ineffective in the halo. Because of the magnetic turbulence inside the disc, 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 disc, 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 per cent, 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 broad-band polarimetry such as that with the Square Kilometre Array is achieved.
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