Magnetic Fields in the Shapley Supercluster Core with POSSUM: Challenging Model Predictions

TL;DR

The paper reports a significant excess RM scatter in the Shapley Supercluster Core detected with a dense POSSUM RM grid and Planck tSZ data, constraining a magnetic field strength of roughly 1–3 μG in SSC-scale gas. It compares the observations to Gaussian random-field models (MiRò) and to cosmological MHD simulations (SLOW), finding that the observed RM-scatter profile is flatter than model predictions and favors weak dependence of |B| on n_e (η<0.5) and turbulent amplification in the intercluster medium. MiRò with B_rms ≈ 2.5 μG and η ≈ 0 provides the best fit among the Gaussian-field realizations, while SLOW’s turbulent-amplification scenario (B_𝓕 ∝ n_e^{1/2} v_{turb}) best matches the data among the cosmological runs, though there remains a discrepancy at larger radii. The results reveal a challenge in reconciling flat RM profiles with even the most realistic simulations, highlighting the need for denser RM grids and improved tSZ maps to unlock the physics of magnetic fields in interfacing cluster environments and filaments.

Abstract

Faraday Rotation Measure (RM) Grids provide a sensitive means to trace magnetized plasma across a wide range of cosmic environments. We study the RM signal from the Shapley Supercluster Core (SSC), in order to constrain the magnetic field properties of the gas. The SSC region consists of two galaxy clusters A3558 and A3562, and two galaxy groups between them, at $z\simeq 0.048$. We combine RM Grid data with thermal Sunyaev-Zeldovich effect data, obtained from the POSSUM pilot survey, and Planck, respectively. To robustly determine the gas density, its magnetic field properties, and their correlation, we study the RM scatter in the SSC region and its behavior as a function of distance to the nearest cluster/group. We compare observational results with semi-analytic Gaussian random field models and more realistic cosmological MHD simulations. With a sky-density of 36 RMs/deg$^{2}$, we detect an excess RM scatter of $30.5\pm 4.6 \, \mathrm{rad/m^2}$ in the SSC region. Comparing with models, we find an average magnetic field strength of 1-3 $μ$G (in the groups and clusters). The RM scatter profile, derived from data ranging from 0.3-1.8 $r_{500}$ for all objects, is systematically flatter than expected compared to models, with $η<0.5$ 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 in the intercluster regions on scales $\lesssim 0.8\,r_{500}$. 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 $r_{500}$. 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.

Figures (15)

• Figure 1: In this figure we aim to show in a clear and visual way the POSSUM $\text{RM}$s used in this work and the definition of the different $\text{RM}$ sub-samples. The background shows the Planck tSZ effect $y$-map. The dash-dotted squares represent the (6 deg)$^2$ Band 1 ASKAP fields, namely, the "core" and "south" fields, with their centers represented by the red crosses. The black contour represents the $y_{\text{bdry}}=4.24\times 10^{-6}$ value used to define the on-target (orange triangles) and off-target (purple dots) samples. These two are inside the 1.71$\times$2.42 deg$^2$ (5.8$\times$8.2 Mpc$^2$) $y$-map cutout used for the analysis (see Sect. \ref{['sect:planck']}). The zoomed-in region at the top left corner of the plot shows the bridge (cyan triangles) and clusters (blue triangles) sub-samples, all of which belong in the on-target region. The bridge box used to define the bridge sources is also represented to ease the interpretation of the plot.
• Figure 2: $\text{RRM}$ and $\text{GRM}$ maps made by interpolation to the nearest pixel of the estimated value at the particular position of a source from our catalog with the annulus method. The black contour corresponds to $y_{\text{bdry}}=4.24\times 10^{-6}$ (same as in Fig. \ref{['fig:subsamples']}, see Section \ref{['sect:subsamples']}). The centers of the clusters and groups are represented by the black crosses, while the dash-dotted circles represent their $r_{500}$. Left: Residual rotation measure map. The random nature of the patches in the map with similar sizes between them indicates that we have removed larger coherent RM structure of the foreground Galactic contribution, while retaining the information about the SSC. Right: $\text{GRM}$ map. Opposite to the behavior of the patches in the $\text{RRM}$ map, this map exhibits a large continuous gradient in the RMs, expected for the large scale Galactic contribution.
• Figure 3: Histograms of observed rotation measures ($\text{RM}_{\text{obs}}$) and residual rotation measures ($\text{RRM}$) of the 149 sources used for our analysis.
• Figure 4: tSZ Planck map of the A3558-A3562 clusters system as well as the two massive groups of galaxies SC 1327, SC1329. The triangles represent the locations of the background ASKAP radiogalaxies, and they are colored by their $\lvert\text{RRM}\rvert$ values. We have also represented the $r_{500}$ of the four objects as reference for the size of the system at the plane of the sky, along with their centers. The black contour represents the threshold we have used to define the boundary between the off-target and on-target regions: $y_{\text{bdry}}=2y_{\text{rms}}=4.24\times 10^{-6}$. The rectangular box defines the bridge. The sources inside it sample the region between the Abell clusters and outside their $r_{500}$, despite some overlapping effects. The counterpart on-target sources that lie outside the box, mainly sample the inside the of $r_{500}$ of the clusters, thus its name: the clusters sub-sample.
• Figure 5: Observed $\mathfrak{S}_{\text{RM}}(d_{\text{nrst}})$ profile (black line). The gray shaded region is the 68$\%$ confidence level (C.L.) error region. The blue crosses represent the amplitude of the $\text{RRM}$s of the on-target region, while the horizontal line is $\sigma_{\text{off-target}}$. The arrow-shaped data point corresponds to an outlier value of $\sim230$ rad/m$^2$. The vertical line is set at $0.7\,r_{500}$, which is the scale at which overlapping effects start to take place. The horizontal segment at the lower left corner represents the median window size of $\sim0.3\,r_{500}$. The vertical purple error bar (lower right corner) indicates the median on-target $\delta\text{RRM}=4.6$rad/m$^2$.