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

Abstract

Context. Radio relics are megaparsec-sized synchrotron sources located in the outskirts of some merging galaxy clusters. Binarymerging systems with a favorable orientation may host two almost symmetric relics, named double radio relics. Aims. 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. Methods. 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, and 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, the distance from the cluster center, and the shock Mach number. Results. 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., restframe 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.

Cosmic-Ray Acceleration and Magnetic Fields in Galaxy Clusters and Beyond: Insights from Radio Observations

“…In most cases, radio relics show a high degree of polarisation, whose origin was uncertain for a long time (e.g., [162,175,213,214,[217][218][219] and references therein). The two competing scenarios were that the degree of polarisation is caused either by a magnetic field that is ordered on large scales, or by the compression and alignment of a magnetic field that is tangled on small scales [220].…”

Star formation as a function of environment in the MeerKAT Galaxy Clusters Legacy Survey

“…Knowles et al (2022) reported the detection of a new candidate relic (see Figure 4.1) in the North West direction of the cluster which is smaller and fainter than the previously observed South East relic. The Abell 521 relic pair has a configuration in the North-West South-East direction and has been noted as a double relic system (Stuardi et al, 2022). The Abell 521 South East relic spectral index has been estimated to be −1.45±0.02 between the 153 MHz and 5 GHz frequency range (Macario et al, 2013).…”