Magnetic fields in the Shapley Supercluster core with POSSUM: Challenging model predictions

¹ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM, Universidad Complutense de Madrid 28040 Madrid, Spain
² Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna Via P. Gobetti 93/2 40129 Bologna, Italy
³ INAF – Istituto di Radioastronomia Via P. Gobetti 101 40129 Bologna, Italy
⁴ Department of Astronomy and Astrophysics, The University of Chicago Chicago IL 60637, USA
⁵ Dunlap Institute for Astronomy and Astrophysics, University of Toronto 50 St. George Street Toronto M5S 3H4 ON, Canada
⁶ Research School of Astronomy & Astrophysics, The Australian National University Canberra ACT 2611, Australia
⁷ National Research Council Canada, Herzberg Research Centre for Astronomy and Astrophysics, Dominion Radio Astrophysical Observatory PO Box 248 Penticton BC V2A 6J9, Canada
⁸ Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
⁹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München Scheinerstr.1 81679 München, Germany
¹⁰ Max-Planck-Institut für Astrophysik Karl-Schwarzschild-Str. 1 85748 Garching, Germany
¹¹ U.S. Naval Research Laboratory 4555 Overlook Ave. SW Washington DC 20375, USA
¹² David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto 50 St. George Street Toronto M5S 3H4 ON, Canada
¹³ Max-Planck-Institut für Radioastronomie Auf dem Hügel 69 53121 Bonn, Germany
¹⁴ Minnesota Institute for Astrophysics, University of Minnesota 116 Church Street SE Minneapolis MN 55455, USA
¹⁵ Trottier Space Institute, McGill University 3550 University Street Montreal QC H3A 2A7, Canada

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 July 2025
Accepted: 17 November 2025

Abstract

Context. Faraday rotation measure (RM) grids provide a sensitive means to trace magnetized plasma across a wide range of cosmic environments.

Aims. We study the RM signal from the Shapley Supercluster core (SSC) in order to constrain the magnetic field properties of its gas. The SSC region consists of two galaxy clusters, A3558 and A3562, and two galaxy groups between them, at z ≃ 0.048.

Methods. We combined RM Grid data with thermal Sunyaev-Zeldovich effect data, obtained from the POlarisation Sky Survey of the Universe’s Magnetism (POSSUM) pilot survey, and Planck, respectively. To robustly determine the gas density, its magnetic field properties, and their correlation |B| ∝ n_e^η, we studied the RM scatter in the SSC region (𝔖_RM) and its behavior as a function of distance to the nearest cluster and/or group (d_nrst). We compared observational results with semi-analytic Gaussian random field models and more realistic cosmological magnetohydrodynamical (MHD) simulations.

Results. With a sky-density of 36 RMs/deg², we detect an excess RM scatter of 30.5 ± 4.6 rad/m² in the SSC region. When we compare with models, we find an average magnetic field strength of ∼1−3 μG (in the groups and clusters). The 𝔖_RM(d_nrst) profile, derived from data ranging from ∼0.3−1.8 r₅₀₀ for all objects, is systematically flatter than expected compared to the models, with η < 0.5 being favored. Despite this discrepancy, we find that cosmological MHD simulations matched to the SSC structure most closely align with scenarios where the magnetic field is amplified by the turbulent velocity (v_turb) in the intercluster regions B_ℱ ∝ n_e^1/2v_turb on scales d_nrst ≲ 0.8.

Conclusions. The dense RM grid and precision provided by POSSUM allows us to probe magnetized gas in the SSC clusters and groups on scales within and beyond their r₅₀₀. Flatter-than-expected RM scatter profiles reveal a significant challenge in reconciling observations with even the most realistic predictions from cosmological MHD simulations in the outskirts of interacting clusters.