The line-of-sight magnetic field of galaxy clusters can be probed using Faraday rotation measure (RM) data. However, our understanding of cluster magnetism is limited due to the scarcity of polarized background radio sources, with most previous studies being constrained to ∼10 sources per cluster. Leveraging the increased source density of the ASKAP POSSUM (POlarisation Sky Survey of the Universe’s Magnetism) Survey, Khadir et al. probe the magnetic field properties of the galaxy cluster Abell 3581 (A3581) with 111 RMs. They find that the standard deviation in the RM declines monotonically with increasing radius up to 0.75 Mpc, agreeing with a radially declining magnetic field and electron density profile modeled as Gaussian and lognormal random fields, respectively. For the first time, they compare the observed RMs in a cluster to full magnetohydrodynamic simulated clusters and find that the non-monotonic trend in RM standard deviation past 0.75 Mpc in A3581 is likely caused by past or present merger activity. They identify a possible candidate for a merger to be a nearby galaxy group, which would be the first group detected in RMs that is not strongly emitting in X-rays.

The image above shows locations and values of the RRMs (residual rotation measures — the rotation measure after the contribution of the Galactic RM has been subtracted from the observed RM) that are within the nominal extent (marked with a dashed circle) of A3581.  At a redshift of 0.022, the radius of A3581 is 925 kiloparsecs.  The colour bars represent the RRMs. The plus sign shows the location of the center of RM, and the dashed line portrays the axis of symmetry.