Radio broadband visualization of global three-dimensional magnetohydrodynamical simulations of spiral galaxies - I. Faraday rotation at 8GHz

Observational study of galactic magnetic fields is hampered by the fact that the observables only probe various projections of the magnetic fields. Comparison with numerical simulations is helpful to understand the real structures, and observational visualization of numerical data is an important task. In this paper, we investigate 8 GHz radio synchrotron emission from spiral galaxies, using the data of global three-dimensional magnetohydrodynamic simulations. We assume a frequency independent depolarization in our observational visualization. We find that the appearance of the global magnetic field depends on the viewing angle: a face-on view seemingly has hybrid magnetic field types combining axisymmetric modes with higher order modes; at a viewing angle of ~70°, the galaxy seems to contain a ring-like magnetic field structure; while in edge-on view, only field structure parallel to the disc can be seen. The magnetic vector seen at 8 GHz traces the global magnetic field inside the disc. These results indicate that the topology of global magnetic field obtained from the relation between azimuthal angle and Faraday depth strongly depends on the viewing angle of the galaxy. As one of the examples, we compare our results at a viewing angle of 25° with the results of IC342. The relation between azimuthal angle and Faraday depth of the numerical result shows a tendency similar to IC342, such as the peak values of the Faraday depth.