2026A&A...705A.143A

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 (Н''—_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 Н''—_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.