🔭 astrophysics

Magnetic properties of the Abell 3391-3395 system revealed using wide-field MeerKAT polarimetry

Using wide-field MeerKAT polarimetry to construct a dense Faraday rotation measure grid, this study reveals that the Abell 3391-3395 intercluster bridge hosts a magnetic field that is relatively ordered on ~10 kpc scales but less coherent on larger scales, while also establishing improved upper limits on diffuse synchrotron emissivity.

Original authors: V. Gustafsson, M. Brüggen, C. Tasse, S. P. O'Sullivan

Published 2026-04-22

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CC BY 4.0

Original authors: V. Gustafsson, M. Brüggen, C. Tasse, S. P. O'Sullivan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, invisible ocean. We know there are islands in this ocean (galaxies) and massive continents (galaxy clusters), but the water between them—the vast, empty spaces—is mostly invisible to our eyes. Scientists have long suspected that this "intergalactic ocean" isn't empty; it's filled with invisible magnetic fields, like invisible currents or magnetic threads weaving through the cosmos.

This paper is about a team of astronomers trying to map those invisible threads in a specific neighborhood of the universe: a pair of colliding galaxy clusters called Abell 3391 and Abell 3395.

Here is the story of how they did it, explained simply:

1. The Challenge: Seeing the Invisible

The problem is that the magnetic fields in the space between galaxies are very weak. They don't glow like stars, so standard telescopes can't see them directly. It's like trying to see a gentle breeze in a dark room; you can't see the wind itself, but you can see how it moves the leaves on a tree.

To "see" the magnetic fields, the astronomers used a clever trick called Faraday Rotation.

The Analogy: Imagine shining a flashlight through a piece of stained glass. The glass twists the light slightly. If you know how much the light twists, you can figure out how thick or strong the glass is.
In Space: When light from a distant galaxy (the flashlight) travels through the magnetic fields of our target (the glass), its polarization (the direction the light waves vibrate) gets twisted. By measuring how much the twist happened, the team could calculate the strength and structure of the magnetic fields in the space between the clusters.

2. The Tool: The "MeerKAT" Eye

To do this, they used the MeerKAT radio telescope in South Africa. Think of MeerKAT as a giant, highly sensitive ear that listens to radio waves instead of sound.

They didn't just take one picture; they took a massive "mosaic" (like a giant jigsaw puzzle) covering a huge patch of sky.
They used advanced computer techniques to clean up the data, removing static and interference, much like a sound engineer removing background noise from a recording to hear a whisper clearly.

3. The Discovery: A Map of Magnetic Twists

The team found 434 distant galaxies acting as their "flashlights." By measuring the twist in the light from all these galaxies, they built a dense map of the magnetic fields.

Here is what they found in different parts of the map:

Inside the Galaxy Clusters (The Stormy Seas):
Inside the two main clusters, the magnetic fields were chaotic and turbulent. The "twist" in the light varied wildly from one galaxy to the next.
- Analogy: Imagine standing in a hurricane. The wind is blowing in every direction, changing rapidly. This makes sense because galaxy clusters are violent places where gas is swirling and colliding.
The Bridge Between Them (The Calm Corridor):
Between the two clusters, there is a "bridge" of hot gas connecting them. The astronomers expected this bridge to be just as chaotic as the clusters.
- The Surprise: Instead, the magnetic fields in the bridge were surprisingly ordered and calm. The "twist" in the light was very consistent.
- Analogy: Imagine the space between the two clusters is like a long, straight hallway. While the rooms at either end are chaotic parties, the hallway itself is quiet and tidy. The magnetic fields here seem to be stretched out and aligned, perhaps because the two clusters are slowly pushing toward each other, smoothing out the magnetic threads like a comb running through hair.
The "Ghost" Bridge:
The team also looked for glowing radio light (synchrotron emission) in that bridge, which would indicate high-energy particles. They found nothing.
- The Result: They set a new, very strict limit on how much "glow" could be there. It's like saying, "We looked for a ghost in the hallway, and if it's there, it's so faint we can't even imagine it." This tells us that the bridge is magnetized but doesn't have the energetic fireworks seen in other parts of the universe.

4. Why This Matters

This discovery is a big deal for a few reasons:

It proves the method works: They successfully mapped magnetic fields in a place where no one could see them directly before.
It reveals the "Pre-Merger" state: The fact that the bridge is calm suggests the two galaxy clusters haven't fully crashed into each other yet. They are in the "pre-collision" phase, where the gas is being squeezed and smoothed out before the big impact.
It helps us understand the Universe: Magnetic fields are the "glue" and the "traffic controllers" of the cosmos. Understanding how they behave in these vast, empty spaces helps us understand how the universe formed and how galaxies grow.

Summary

In short, this paper is a story about using twisted light to map invisible magnetic threads in the space between two colliding galaxy giants. They found that while the giants themselves are chaotic, the space connecting them is surprisingly orderly, offering a rare glimpse into the quiet moments before a cosmic collision.

1. Problem and Motivation

Magnetic fields in the outskirts of galaxy clusters and the intercluster medium (ICM) remain poorly constrained due to the difficulty of detecting diffuse synchrotron emission, which often has extremely low surface brightness. While Faraday Rotation Measures (RMs) of polarized background sources offer a statistical method to probe these magnetized environments, previous surveys often lacked the sensitivity or source density required for robust analysis in specific, complex systems.

The Abell 3391 and Abell 3395 (A3391/95) system is a nearby ( $z \approx 0.053$ ) interacting pair of galaxy clusters connected by a well-established X-ray bridge. This system is a prime target for investigating magnetization in the intercluster environment, yet previous radio observations (e.g., with ASKAP) yielded only upper limits on diffuse emission and lacked a dense RM grid. The authors aim to characterize the magnetized environment of A3391/95 and its surroundings by constructing a high-density RM grid using deep, wide-field polarimetry.

2. Methodology

Observations:

Instrument: MeerKAT radio telescope.
Setup: A three-pointing mosaic (central field + two 25' offsets) observed over three epochs (Dec 2022 – Feb 2023), totaling 18 hours on source.
Band: L-band (856 MHz centered at 1284 MHz) with 4096 channels.
Auxiliary Data: eROSITA X-ray images (0.2–2.3 keV) were used to define on-target (cluster/bridge) and off-target regions via wavelet filtering.

Data Reduction and Calibration Strategy:
The authors employed an advanced reduction pipeline to achieve high sensitivity and minimize artifacts across the wide field of view (3° × 2°):

Cross-Calibration: Performed using the CARACal pipeline with standard calibrators for flux, bandpass, and polarization.
Scalar Direction-Independent Self-Calibration: Initial models were built using ddfacet (Högbom mode) followed by scalar phase and complex gain self-calibration using quartical.
Visibility Plane Mosaicing: Instead of imaging individual pointings, visibilities were shifted to a common phase center and gridded onto a single $uv$-plane before deconvolution. This improved dynamic range by ~50% compared to individual images.
Direction-Dependent (DD) Calibration: Crucial for handling bright, extended sources near the primary beam edge. The killms software was used with a non-linear Kalman Filter (KAFCA) to solve for scalar gains in 32 frequency bins across 10 solution directions.
Full Jones Calibration & Ionospheric Correction:
- Ionospheric Faraday rotation was corrected using spinifex (based on satellite TEC data).
- Full Stokes ( $I, Q, U, V$ ) imaging was performed using Faraday synthesis in ddfacet.
- A unique approach was taken regarding the Primary Beam (PB): The PB-corrupted 3D Point Spread Function (PSF) was used during deconvolution to maintain flat noise levels across the field, with PB correction applied only after deconvolution.
- A 3D automask based on $Q$ and $U$ residuals was used to speed up deconvolution.

Analysis Techniques:

RM Grid Construction: 434 polarized sources were identified. RMs were derived via Faraday synthesis, with Galactic Rotation Measure (GRM) removed using the median annulus method.
Statistical Analysis:
- RM Scatter: Compared the scatter of RMs in on-target (clusters/bridge) vs. off-target regions.
- Structure Function: Used the RM structure function ( $D_{RM}$ ) to compare fluctuations on matched angular scales between the bridge and off-target regions.
- Depolarization: Calculated the depolarization ratio ( $DP = p_{low}/p_{high}$ ) between two subbands to probe Faraday complexity on $\sim 10$ kpc scales.
Diffuse Emission Search: Searched for diffuse synchrotron emission in the bridge after subtracting compact sources and performing low-resolution imaging.

3. Key Contributions

Methodological Advancement: Demonstrated a robust pipeline combining visibility-plane mosaicing, direction-dependent calibration, and Faraday synthesis to produce deep, wide-field full-Stokes images with minimal artifacts.
Dense RM Grid: Generated a high-density RM grid (field-averaged 73 sources deg $^{-2}$ , peaking at 110 sources deg $^{-2}$ in the center), significantly exceeding previous surveys like POSSUM pilot data.
Differentiated Analysis: Successfully separated the magnetic properties of the cluster cores, the intercluster bridge, and the surrounding field, revealing distinct behaviors in each region.

4. Key Results

RM Scatter and Enhancement:

Clusters: The cluster regions (A3391 and A3395) show a statistically significant enhancement in RM scatter compared to the off-target control region.
- On-target scatter: $29.31 \pm 1.26$ rad m $^{-2}$ .
- Off-target scatter: $18.86 \pm 0.69$ rad m $^{-2}$ .
- Excess scatter: $22.42 \pm 1.73$ rad m $^{-2}$ (13 $\sigma$ significance).
- Specific to clusters: A3391 ( $\sigma_{RM} \approx 35.9$ ) and A3395 ( $\sigma_{RM} \approx 39.3$ ) show the highest values, consistent with turbulent intracluster magnetic fields.
Intercluster Bridge: Surprisingly, the bridge region shows a low RM scatter ( $13.98 \pm 1.64$ $13.98 \pm 1.64$ rad m $^{-2}$ $^{- 2}$ ), which is actually lower than the off-target scatter.
- However, an RM structure function analysis on matched angular scales hints at larger RM differences in the bridge compared to off-target, though not statistically significant ( $p=0.11$ ).

Depolarization:

A3391: Shows strong depolarization ($DP = 0.59$), indicating high Faraday complexity within the cluster.
Bridge: Shows a depolarization ratio ($DP = 0.78$) comparable to the off-target sample ($0.74$). This suggests the bridge does not introduce strong additional depolarization on $\sim 10$ kpc scales.

Diffuse Emission:

No diffuse synchrotron emission was detected in the bridge.
The study sets a new upper limit on radio emissivity: $\langle J \rangle_{1 \text{GHz}} < 1.83 \times 10^{-45}$ W Hz $^{-1}$ m $^{-3}$ . This is nearly an order of magnitude lower than previous ASKAP limits, thanks to MeerKAT's superior sensitivity.

Galactic Foreground:

The Galactic RM correction was effective, removing large-scale gradients. Residual correlations with HI column density suggest a small remaining Galactic component, but the analysis confirms the RM variations are dominated by extragalactic structures.

5. Significance and Interpretation

Ordered Magnetic Fields in the Bridge: The combination of low RM scatter, low depolarization, and the hint of larger-scale structure suggests that the magnetic field in the A3391/95 bridge is relatively ordered on scales of $\sim 10$ kpc but potentially more structured on larger scales.
Pre-Merger State: These findings support the interpretation that A3391/95 is in a pre-merger phase. Theoretical MHD simulations suggest that pre-merger compression can align magnetic fields in the midplane between clusters, reducing line-of-sight fluctuations and turbulence. This is consistent with recent XRISM observations showing calm gas motions ( $\sim 100$ km/s turbulence) in A3395S.
Contrast with Other Systems: Unlike the A399-A401 system, where a diffuse radio bridge is detected, A3391/95 shows no such emission. This implies that particle acceleration mechanisms (shocks/turbulence) capable of generating synchrotron emission have not yet been fully activated in the A3391/95 bridge, likely because the merger has not reached the core-passage stage.
Future Prospects: The study highlights the power of wide-field polarimetry for mapping cosmic web magnetism. The results suggest that while the bridge is magnetized, the field is currently too ordered or the electron density too low to produce detectable diffuse synchrotron emission, requiring even deeper RM sampling to resolve the larger-scale magnetic structure.