Turbulent magnetic fields in the merging galaxy cluster MACS J0717.5+3745: polarization analysis

TL;DR

This study uses wideband polarimetric VLA observations to analyze the magnetic field structure and Faraday effects in the complex radio relic and halo system of the merging galaxy cluster MACS J0717.5+3745, revealing detailed magnetic field properties and depolarization mechanisms.

Contribution

It provides a detailed polarization and Faraday rotation analysis of the radio relic and halo in MACS J0717.5+3745, demonstrating the magnetic field structure and the impact of the intracluster medium.

Findings

Relic is highly polarized with some regions exceeding 30% polarization.

Faraday distribution varies across the relic, indicating complex magnetic environments.

Polarized emission in the halo likely originates from shocks, not the halo itself.

Abstract

We present wideband polarimetric observations, obtained with the Karl G. Jansky Very Large Array (VLA), of the merging galaxy cluster MACS J0717.5+3745, which hosts one of the most complex known radio relic and halo systems. We use both Rotation Measure Synthesis and QU-fitting, and find a reasonable agreement of the results obtained with these methods, in particular, when the Faraday distribution is simple and the depolarization is mild. The relic is highly polarized over its entire length reaching a fractional polarization ${>}30\%$ in some regions. We also observe a strong wavelength-dependent depolarization for some regions of the relic. The northern part of the relic shows a complex Faraday distribution suggesting that this region is located in or behind the intracluster medium (ICM). Conversely, the southern part of the relic shows a Rotation Measure very close to the Galactic foreground, with a rather low Faraday dispersion, indicating very little magnetoionic material intervening the line-of-sight. From spatially resolved polarization analysis, we find that the scatter of Faraday depths correlates with the depolarization, indicating that the tangled magnetic field in the ICM causes the depolarization. At the position of a well known narrow-angle-tailed galaxy (NAT), we find evidence of two components clearly separated in Faraday space. The high Faraday dispersion component seems to be associated with the NAT, suggesting the NAT is embedded in the ICM while the southern part of the relic lies in front of it. The magnetic field orientation follows the relic structure indicating a well-ordered magnetic field. We also detect polarized emission in the halo region; however the absence of significant Faraday rotation and a low value of Faraday dispersion suggests the polarized emission, previously considered as the part of the halo, has a shock(s) origin.