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

TL;DR

This study leverages a dense RM grid from the POSSUM survey to map the line-of-sight magnetic field in Abell 3581 via RM scatter, comparing observations to Gaussian-random and lognormal ICM models and to full MHD simulations from the TNG-Cluster project. RM-synthesis and QU-fitting classify Faraday complexity, while Galactic RM corrections and careful cluster membership filtering yield 111 background RRMs for intrinsic cluster analysis. The inner 0.75 Mpc is well described by moderate central fields with B0 in the microgauss range depending on the density scaling, but the outskirts show non-monotonic RM scatter likely driven by merger-related substructure and a clump associated with a nearby galaxy group. The work demonstrates the power of high-density RM grids for probing cluster magnetism, highlights the role of mergers in shaping RM statistics, and sets the stage for future SKA-era polarization surveys.

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 $θ= 52\pm 4$ deg.

Figures (21)

• Figure 1: (a) The locations and values of the observed RMs across the whole POSSUM field, along with identified nearby clusters. The black circle indicates $2R_{500}$ for A3581 and the green circles indicate $2R_{500}$ for nearby clusters. (b) The locations and values of the RRMs that are within $2R_{500}$ (marked with a dashed circle) of A3581. The color bars represent the RRMs saturated from $-40$ rad m$^{-2}$ to $+40$ rad m$^{-2}$. The plus sign shows the location of the center of RM, and the dashed line portrays the axis of symmetry (see Section \ref{['sec:asymmetry']} for further details).
• Figure 2: (a) The RRMs as a function of the projected distance to the X-ray centroid. The black line indicates the $2R_{500}$ radius using the value from 2024ApJS..272...39W, and the red line indicates the $2R_{500}$ radius using the value from the SRG/eROSITA all-sky survey 2024AA...685A.106B. (b) The RRM scatter as a function of the projected distance from the center of the cluster. To calculate the scatter we used a running bin and fixed the number of points per bin to be 20. The median bin width is 0.27 Mpc.
• Figure 3: RRM scatter plots as a function of radius for the best-fit $B_0$ cluster models for $\eta = {0, 0.25, 0.5}$. For each model, we ran ten iterations; the solid red line indicates the median RRM scatter and the shaded region indicates the 1$\sigma$ scatter. The solid blue line is the observed RRM scatter in A3581 as in Figure \ref{['fig:rrm_scatter']}, with the shaded region indicating the uncertainty. The red dotted line indicates $r = 0.75$ Mpc, beyond which we do not compute the overlap metric (Eq. \ref{['eq:overlap']}).
• Figure 4: (a) Comparison of the RM scatter in A3581 (in blue) and three simulated analogues (in red) of separate clusters that were found in TNG-Cluster. The different simulated clusters are indicated by different line styles. (b) The RMs of the closest analogs to A3581 from TNG-Cluster.
• Figure 5: Logarithm of the LOS magnetic field strength for the CC simulated analogue cluster from the TNG-Cluster simulation. The dotted circle represents $2R_{500}$ and the solid circles represent the positions of the sampled RMs (obtained from the observation of A3581).