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

Abstract

We present wideband (1−6.5 GHz) 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 (850 kpc), 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. We conclude that the ICM magnetic field could be highly turbulent. 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. If true, this implies that the relic and this radio galaxy are not necessarily physically connected and thus, the relic may be not powered by the shock re-acceleration of fossil electrons from the NAT. 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.

On the Polarisation of Radio Relics

“…The depolarisation is affected by the amount of ICM along the line-of-sight, see e.g. the discussion in [13]. When correcting for the Faraday rotation along the line-of-sight one finds the intrinsic polarisation angle of the synchrotron emission.…”

On the Polarisation of Radio Relics

“…For several relics the magnetic field which corresponds to polarisation direction (i.e., the so-called polarisation B-vector) is astonishingly well aligned with the relic morphology, i.e., the orientation of the shock surface, most spectacularly for the Sausage relic [11,14]. Other relics which show this alignment are for instance the Toothbrush [9], the northern relic in PSZ1 G096.89 + 24.17 [15] and the relic in MACS J0717.5 + 3745 [13]. The orientation of the polarisation has revealed that the magnetic field is well aligned with the surface of the shock.…”

“…Some relics show a strong frequency dependent depolarisation which can be attributed to the Faraday depth dispersion due to the magnetoionic intra-cluster medium (ICM) along the line-of-sight from the observer to the emission region. The depolarisation is affected by the amount of ICM along the line-of-sight, see e.g., the discussion in [13]. When correcting for the Faraday rotation along the line-of-sight one finds the intrinsic polarisation angle of the synchrotron emission.…”

Deep low-frequency radio observations of Abell 2256 I: The filamentary radio relic

“…Galaxy clusters undergoing mergers often show spectacular, megaparsec-scale radio relics and radio halos (see van Weeren et al 2019, for a recent review). The radio spectra of such sources are steep (α ≤ −1.0 where Di Gennaro et al 2021;Rajpurohit et al 2020aRajpurohit et al , 2021a and believed to originate from shock fronts generated in the intracluster medium (ICM) when galaxy clusters merge (Ensslin et al 1998;Roettiger et al 1999). This connection has been established by finding X-ray surface brightness and temperature discontinuities at the location of some relics (see e.g., Sarazin et al 2013;Ogrean et al 2013;Shimwell et al 2015;van Weeren et al 2016;Botteon et al 2016;Thölken et al 2018;Di Gennaro et al 2019).…”