Pseudo-observation of spiral galaxies in the radio band to verify depolarization models

Abstract

Studies of the three-dimensional (3D) structures of galactic magnetic fields are now entering a new era, with broadband, highly sensitive radio observations and new analysis methods. To reveal the magnetic field configuration from the observed value integrated along the line of sight, it is necessary to derive an appropriate model that can reproduce the observational characteristics. We aim to clarify the relationship between the radiation field and the spatial distribution of physical quantities through pseudo-observations using global 3D magnetohydrodynamics (MHD) simulation results. In particular, we focus on using the depolarization effect to verify the polarization model and to identify the emission region. First, we show that wavelength-independent depolarization, which takes into account anisotropic turbulence, does not work efficiently because the polarized emission is stronger in regions of ordered spiral fields than in regions dominated by isotropic turbulent fields. Beam depolarization becomes more effective below 1 GHz. Although close to the equatorial plane there will be strong depolarization which increases with observing wavelength, this effect is less in the halo, making halo magnetic fields detectable through their polarized emission at meter-wavelength bands. Although polarized emission from the halo is below the detection limit of current facilities, it will be detectable within the SKA era. In addition, we found that the spiral polarization projected on a screen is produced by overlapping magnetic flux tubes extending to different heights from the equatorial plane. This suggests that the traditional classification of global magnetic fields has difficulty reproducing the global structure of the magnetic fields. Finally, we demonstrate the method to separate magnetic flux tubes at different heights by using peak frequencies which cause the decreasing of polarized flux.