Revealing the magnetization of the intracluster medium of Abell 3581 using background Faraday rotation measures from the POSSUM survey

TL;DR

This study uses enhanced background Faraday rotation measures from the POSSUM survey to analyze the magnetic field structure of galaxy cluster Abell 3581, revealing a radially declining magnetic field and evidence of merger activity.

Contribution

It provides the first detailed RM-based magnetic field analysis of A3581 with 111 sources, comparing observations to models and simulations, and identifying merger signatures.

Findings

RM standard deviation decreases with radius up to 0.75 Mpc

Observed RMs match models with radially declining magnetic fields

Evidence suggests past or ongoing merger activity in A3581

Abstract

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 $\sim 10$ sources per cluster. Leveraging the increased source density of the POlarisation Sky Survey of the Universe's Magnetism (POSSUM), we probe the magnetic field properties of the galaxy cluster Abell 3581 {(A3581)} with 111 RMs. We 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. {We compare our observations of the inner 0.75 Mpc of A3581 to various semi-analytic models of the magnetic field and electron density, and obtain several best-fit models.} For the first time, we compare the observed RMs in a cluster to full magnetohydrodynamic simulated clusters from TNG-Cluster 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. We identify a possible candidate for a merger to be the galaxy group [DZ2015b] 276, which would be the first group detected in RMs that is not strongly emitting in X-rays. We find a possible merger axis of A3581 with this group at a position angle of $\theta = 52\pm 4$ deg.