Using the polarization properties of double radio relics to probe the turbulent compression scenario

TL;DR

This study uses polarization and Faraday rotation data from four galaxy clusters with double radio relics to test models of turbulent magnetic fields and shock wave propagation in merging galaxy clusters.

Contribution

It provides new polarization measurements for four clusters and compares observed properties with magneto-hydrodynamical simulations, supporting the turbulent compression scenario.

Findings

Weak correlations between polarization and physical parameters are consistent with turbulent magnetic fields.

Faraday rotation properties suggest shock waves with Mach numbers > 2.5 propagate in a turbulent intra-cluster medium.

External and internal Faraday depolarization significantly influence relic polarization at 1.4 GHz.

Abstract

Radio relics are Mpc-size synchrotron sources located in the outskirts of some merging galaxy clusters. Binary-merging systems with favorable orientation may host two almost symmetric relics, named double radio relics. Double radio relics are seen preferentially edge-on and, thus, constitute a privileged sample for statistical studies. Their polarization and Faraday rotation properties give direct access to the relics origin and magnetic fields. In this paper, we present a polarization and Rotation Measure (RM) synthesis study of four clusters hosting double radio relics, namely 8C 0212+703, Abell 3365, PLCK G287.0+32.9, previously missing polarization studies, and ZwCl 2341+0000, for which conflicting results have been reported. We used 1-2 GHz Karl G. Jansky Very Large Array observations. We also provide an updated compilation of known double radio relics with important observed quantities. We studied their polarization and Faraday rotation properties at 1.4 GHz and we searched for correlations between fractional polarization and physical resolution, distance from the cluster center, and shock Mach number. The weak correlations found between these quantities are well reproduced by state-of-the-art magneto-hydrodynamical simulations of radio relics, confirming that merger shock waves propagate in a turbulent medium with tangled magnetic fields. Both external and internal Faraday depolarization should play a fundamental role in determining the polarization properties of radio relics at 1.4 GHz. Although the number of double radio relics with RM information is still low, their Faraday rotation properties (i.e., rest-frame RM and RM dispersion below 40 rad m$^{-2}$ and non-Gaussian RM distribution) can be explained in the scenario in which shock waves with Mach numbers larger than 2.5 propagate along the plane of the sky and compress the turbulent intra-cluster medium.