POSSUM RM grid of Shapley Supercluster core

We compared observational results with semi-analytic Gaussian random field models and more realistic cosmological magnetohydrodynamical (MHD) simulations. With a sky-density of 36RMs/deg^2^, we detect an excess RM scatter of 30.5+/-4.6rad/m^2^ in the SSC region. When we compare with models, we find an average magnetic field strength of ~1-3 uG (in the groups and clusters).

The G_RM_(dnrst) profile, derived from data ranging from ~0.3-1.8r500 for all objects, is systematically flatter than expected compared to the models, with eta<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 BF{prop.to}n_e_^1/2^*v_turb_ on scales d_nrst_<~0.8.

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.