Radio broad-band visualization of global three-dimensional magnetohydrodynamical simulations of spiral galaxies – II. Faraday depolarization from 100 MHz to 10 GHz

Abstract

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 100 MHz 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 disk and the dispersion of Faraday depth becomes about 4 rad m −2 for face-on view and 40 rad m −2 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.

Strategy to Explore Magnetized Cosmic Web with Forthcoming Large Surveys of Rotation Measure

“…Such a source is not suited to the study of WHIM's RM. We expect that it exhibits a strong depolarization feature caused by a turbulent magnetic field [49,50], so that we could discard it from the analysis.…”

Faraday Depolarization Effects in Spiral Galaxies

“…Observational visualization is a powerful way of presenting information of the relation between the FD and the distribution of the physical gas and magnetic-field distributions. Machida et al (2018a) (Reference [6], hearafter Paper I) and Machida et al (2018b) (Reference [7], hearafter Paper II) reported a synthetic observational analysis of spiral galaxies using the spatial distribution of physical values used in numerical simulation. They assumed that the radio signal is a synchrotron emission from background radio sources.…”