Using a dense POSSUM RM grid together with Planck thermal SZ data, we probed the magnetised gas in the core of the Shapley Supercluster, one of the densest structures in the local Universe. We detected a clear excess in RM scatter across the region, indicating magnetic field strengths of roughly 1–3 μG in the clusters and groups. Comparing the observations with semi-analytic models and cosmological MHD simulations revealed flatter-than-expected RM profiles in the cluster outskirts, highlighting a tension with current models of magnetic field amplification. These results show that wide-area RM grids can directly trace magnetised gas beyond cluster cores and into the interacting supercluster environment. See Alonso-López, O’Sullivan et al. (2026) for details.
