Probing the magnetised gas distribution in galaxy groups and the cosmic web with POSSUM Faraday Rotation Measures

Abstract We present initial results from the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM), analyzing 22,817 Faraday Rotation Measures (RMs) with median uncertainties of 1.2 rad m−2 across 1,520 square degrees to study magnetised gas associated with 55 nearby galaxy groups (z ≲ 0.025) with halo masses between 1012.5 and 1014.0 M⊙. We identify two distinct gas phases: the Intragroup Medium (IGrM) within 0–2 splashback radii and the Warm-Hot Intergalactic Medium (WHIM) extending from 2 to 7 splashback radii. These phases enhance the standard deviation of residual (i.e. Galactic foreground RM-subtracted) RMs by 6.9 ± 1.8 rad m−2 and 4.2 ± 1.2 rad m−2 , respectively. Estimated magnetic field strengths are several μG within the IGrM and 0.1–1 μG in the WHIM. We estimate the plasma β in both phases, and show that magnetic pressure might be more dynamically important than in the ICM of more massive clusters or sparse cosmic web filaments. Our findings indicate that ’missing baryons’ in the WHIM likely extend beyond the gravitational radii of group-mass halos to Mpc scales, consistent with large-scale, outflow-driven ’magnetised bubbles’ seen in cosmological simulations. We demonstrate that RM grids are an effective method for detecting magnetised thermal gas at galaxy group interfaces and within the cosmic web. This approach complements X-ray and Sunyaev-Zel’dovich effect methods, and when combined with Fast Radio Burst Dispersion Measures, data from the full POSSUM survey — comprising approximately a million RMs — will allow direct magnetic field measurements to further our understanding of baryon circulation in these environments and the magnetised universe.