Effects of Depolarizing Intervening Galaxies on Background Radio Emission I. Global Disk Magnetic Field

TL;DR

This paper models how depolarizing intervening galaxies affect background radio emission, revealing the importance of magnetic field structure, inclination, and impact parameter, with implications for future radio surveys.

Contribution

It introduces theoretical models of DINGs and analyzes their impact on polarization and Faraday rotation, highlighting the significance of magnetic field configuration and redshift dependence.

Findings

Depolarization depends on galaxy inclination and impact parameter.

FDFs show multi-component structures due to RM variations.

DING contribution to RM standard deviation scales as (1+z)^{-2.7}.

Abstract

External galaxies often intervene in front of background radio sources such as quasars and radio galaxies. Linear polarization of the background emission is depolarized by Faraday rotation of inhomogeneous magnetized plasma of the intervening galaxies. Exploring the depolarizing intervening galaxies (DINGs) can be a powerful tool to investigate the cosmological evolution of the galactic magnetic field. In this paper, we investigate the effects of DINGs on background radio emission using theoretical DING models. We find that complex structures of galaxy result in complicated depolarization features and the Faraday dispersion functions (FDFs), but, for the features of depolarizations and FDFs, the global component of magnetic fields is important. We show the simplest results with ring magnetic field in the galactic disk. We find that the degree of depolarization significantly depends on the inclination angle and the impact parameter of the DING. We found that the larger the standard deviation, the more likely it is that depolarization will occur. The FDF represents the RM structure within the beam. The FDF exhibits multi-components due mainly to the RM structure within the beam and the fraction of the DING that covers the background emission (the filling factor). The peak Faraday depth of the FDF is different from the beam-averaged RM of the DING. The Monte-Carlo simulations indicate that DING's contribution to the standard deviation of observed RMs follows $\sigma_{\rm RM} \propto 1/{(1+z)^k}$ with $k \sim 2.7$ and exhibits a steeper redshift dependence than the wavelength squared. DINGs will have a significant impact on RM catalogs created by future survey projects such as the SKA and SKA Precursor/Pathfinder.