Turbulent magnetic fields in the merging galaxy cluster MACS J0717.5+3745^⋆

¹ Dipartimento di Fisica e Astronomia, Universitá di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
e-mail: kamlesh.rajpurohit@unibo.it
² INAF-Istituto di Radio Astronomia, Via Gobetti 101, 40129 Bologna, Italy
³ Thüringer Landessternwarte (TLS), Sternwarte 5, 07778 Tautenburg, Germany
⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁶ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. S.E., Minneapolis, MN 55455, USA
⁷ Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁹ CSIRO Astronomy and Space Science, PO Box 1130 Bentley, WA 6102, Australia
¹⁰ Astronomy and Astrophysics Division, Physical Research Laboratory, Ahmedabad 380009, India
¹¹ Department of Physics, School of Natural Sciences UNIST, Ulsan 44919, Korea
¹² INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
¹³ Department of Physics, New Mexico Tech, Socorro, NM 87801, USA
¹⁴ National Radio Astronomy Observatory, Socorro, NM 87801, USA
¹⁵ US Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA

Received: 30 September 2021
Accepted: 27 October 2021

Abstract

We present wideband (1 − 6.5 GHz) polarimetric observations, obtained with the Karl G. Jansky Very Large Array, of the merging galaxy cluster MACS J0717.5+3745, which hosts one of the most complex known radio relic and halo systems. We used both rotation measure synthesis and QU-fitting to find a reasonable agreement of the results obtained with these methods, particularly 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 along the line of sight. Based on a spatially resolved polarization analysis, we find that the scatter of Faraday depths is correlated 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 that are clearly separated in the 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, in fact, not be 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 detected polarized emission in the halo region; however, the absence of significant Faraday rotation and a low value of Faraday dispersion suggests the polarized emission that was previously considered as the part of the halo does, in fact, originate from the shock(s).