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MeerKAT observations of Abell 1775 and Abell 1795: the discovery of a hadronic radio halo?

This paper presents new MeerKAT and LOFAR observations of the cool-core clusters Abell 1775 and Abell 1795, revealing unexpectedly large radio haloes that challenge the traditional association of such structures with merging systems and suggest alternative particle acceleration mechanisms like hadronic interactions or overlooked turbulence in relaxed clusters.

Original authors: R. J. van Weeren, E. Osinga, G. Brunetti, C. J. Riseley, A. Botteon, R. Timmerman, A. Bonafede, M. Brüggen, R. Cassano, V. Cuciti, D. Dallacasa, F. de Gasperin, J. M. G. H. J. de Jong, F. Gastaldello

Published 2026-01-26

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CC BY 4.0

Original authors: R. J. van Weeren, E. Osinga, G. Brunetti, C. J. Riseley, A. Botteon, R. Timmerman, A. Bonafede, M. Brüggen, R. Cassano, V. Cuciti, D. Dallacasa, F. de Gasperin, J. M. G. H. J. de Jong, F. Gastaldello, K. Knowles, X. Zhang

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, cosmic ocean. In this ocean, there are massive islands made of thousands of galaxies called galaxy clusters. Usually, these islands are either in a state of calm (relaxed) or in the middle of a violent storm (merging).

Scientists have long known that when these islands crash into each other, they create giant, invisible "radio halos"—glowing clouds of energy that can be seen with special radio telescopes. These halos are like the foam on a crashing wave, created by the turbulence of the collision.

However, this new paper looks at two specific islands, Abell 1775 and Abell 1795, which are supposed to be calm, "relaxed" islands with smooth centers. The researchers used two powerful telescopes, MeerKAT (which listens to high-pitched radio sounds) and LOFAR (which listens to low-pitched sounds), to see what's really happening in the deep calm.

Here is what they found, explained simply:

1. The "Ghost" in the Calm (Abell 1795)

The big surprise happened at Abell 1795. Scientists expected to see a small, faint glow near the center (a "mini-halo") because the cluster is calm. Instead, they found a giant radio halo stretching a massive distance (about 1 million light-years across).

The Analogy: Imagine walking into a quiet library and finding a massive, roaring concert happening in the back. You wouldn't expect that in a quiet place.
The Mystery: Usually, these giant "concerts" only happen in crashing, messy clusters. Finding one in a calm cluster is like finding a hurricane in a calm pond.
The Shape: The glow isn't just a blob; it has a spiral shape, like a galaxy's arms, and even connects to a distant "tailed" galaxy nearby. It's as if the calm pond has a hidden whirlpool that has been spinning for billions of years.

Why is this a big deal?
It suggests that maybe our understanding of how these radio halos form is incomplete. Perhaps even "calm" clusters have enough hidden turbulence to keep these halos alive, or maybe the energy comes from a different source entirely—like a "ghost" of past explosions from the central black hole that we couldn't see before.

2. The Two-Faced Cluster (Abell 1775)

The other cluster, Abell 1775, showed a different kind of trick.

The Analogy: Imagine looking at a painting. From far away (using the high-pitched MeerKAT telescope), it looks like a smooth, soft cloud. But if you zoom in with a special filter (the low-pitched LOFAR telescope), the "cloud" turns out to be a messy web of sharp, jagged threads.
The Discovery: The "smooth" glow seen in high frequencies is actually hiding a complex web of ultra-steep, thin filaments that are only visible at low frequencies. It's like realizing a fluffy cloud is actually made of tangled fishing line.

3. The "Ghost Cavity" Connection

In Abell 1795, the researchers also found a strange feature near the center. There is a long, thin stream of cool gas (an X-ray filament) stretching out from the central black hole.

The Discovery: They found a faint, steep-spectrum radio signal tracing this exact path, ending in a small "hole" in the gas.
The Meaning: This proves that the "hole" isn't just empty space; it's a ghost cavity filled with old, aging radio plasma from a past explosion by the central black hole. It's like finding the faint, fading smoke trail of a firework that went off a long time ago, proving exactly where it landed.

4. Why Did We Miss This Before?

The paper suggests that we might have missed these giant halos in calm clusters for a long time because of "noise."

The Analogy: Imagine trying to hear a whisper in a room where a loud radio is playing. The loud radio (the bright central black hole) drowns out the faint whisper (the giant halo).
The Solution: The researchers used new, better techniques to "turn down the volume" on the bright central radio, allowing them to finally hear the faint, giant halo whispering in the background.

The Bottom Line

This paper tells us that the universe is more complex than we thought.

Calm clusters aren't always calm: They can host giant radio halos that we previously thought only existed in violent crashes.
What you see depends on how you look: Some features only appear at certain radio frequencies, hiding complex structures behind smooth appearances.
Old explosions leave traces: We can now see the "ghosts" of ancient black hole outbursts stretching across the galaxy cluster.

The authors conclude that these discoveries might mean we need to rewrite the rules of how these cosmic radio halos are born and sustained, and that we likely have many more "hidden giants" waiting to be found in the calmest parts of the universe.

Technical Summary: MeerKAT observations of Abell 1775 and Abell 1795: the discovery of a hadronic radio halo?

Problem and Context
Giant radio haloes are typically associated with merging galaxy clusters, while mini-haloes are found in relaxed, cool-core systems. The distinction between these populations is generally clear, yet recent observations suggest a more complex picture. A significant challenge in studying cool-core clusters is the presence of bright central Active Galactic Nuclei (AGN) and sharply peaked gas density profiles, which can obscure faint, extended diffuse radio emission. Furthermore, the detection of large-scale haloes in relaxed clusters is complicated by the potential for ultra-steep spectrum emission from AGN-related filaments to contaminate measurements at low frequencies. This study addresses the question of whether large-scale radio haloes exist in dynamically relaxed cool-core clusters and investigates the physical mechanisms (turbulent re-acceleration vs. hadronic interactions) responsible for their emission.

Methodology
The authors present new L-band (903–1655 MHz) observations of the galaxy clusters Abell 1775 ( $z=0.0720$ ) and Abell 1795 ( $z=0.0625$ ) obtained with the MeerKAT telescope. These data are combined with reprocessed LOFAR Two-metre Sky Survey (LoTSS) data at 120–168 MHz.

Data Reduction: The authors utilized an improved calibration pipeline, facetselfcal, originally developed for LOFAR but adapted here for MeerKAT. This involved direction-independent and direction-dependent (DD) calibration to mitigate artifacts caused by bright central sources (specifically 4C +26.42 in Abell 1795). The pipeline included RFI flagging, self-calibration cycles, and the subtraction of compact sources to reveal diffuse emission.
Imaging and Analysis: High-resolution images were generated, followed by the creation of low-resolution images where compact sources were subtracted to highlight extended structures. Spectral index maps (144–1279 MHz) were produced to characterize the emission.
Modeling: The authors performed radial surface brightness profile fitting using a double-exponential model. They also conducted a point-to-point correlation analysis between radio and X-ray surface brightness to test scaling relations. Finally, they modeled the Abell 1795 radio halo surface brightness under a hadronic scenario, assuming cosmic-ray protons interacting with thermal gas to produce secondary electrons.

Key Results

Abell 1775: The cluster hosts a diffuse radio halo with a total extent of $\sim500$ kpc. The emission is asymmetric, fading more slowly to the east. The radial profile reveals two components: a steep inner component and a shallower outer component. While the MeerKAT image shows a smooth halo, the LOFAR image is dominated by ultra-steep spectrum filaments ( $\alpha \approx -3.0$ ) associated with AGN activity, which are nearly invisible at 1.3 GHz. This highlights the risk of spectral contamination when deriving halo properties from low-frequency data alone.
Abell 1795: The authors detect a large-scale radio halo extending to $\sim1$ $\sim 1$ Mpc, comprising an inner component (previously a candidate mini-halo) and an outer component. The total integrated spectral index is $\alpha = -1.08 \pm 0.06$ $α = - 1.08 \pm 0.06$ , which is relatively flat for a relaxed cluster. The emission exhibits a spiral-like morphology and extends beyond the cold front identified in the south.
- AGN Feedback: A new steep-spectrum feature (S3) is detected south of the central AGN (4C +26.42), tracing a 45 kpc X-ray filament and terminating in an X-ray cavity. This suggests the filament and cavity are linked to a past AGN outburst, challenging previous interpretations that the cavity was a "ghost" feature unrelated to current activity.
- Correlation: A tight, nearly linear point-to-point correlation ( $k = 1.08 \pm 0.01$ ) is found between radio and X-ray surface brightness, suggesting the radio emission closely tracks the thermal gas distribution.
Hadronic Modeling: When modeling the Abell 1795 halo under a hadronic scenario, the observed radio profile is best reproduced if the cosmic-ray proton energy density is constant with radius ( $a=0$ ) and the central magnetic field strength is between 1 and 5 $\mu$ G. This model accounts for the flat spectral index and the close radio-X-ray morphological match.

Significance and Claims
The paper claims that the discovery of a $\sim1$ Mpc radio halo in the relaxed cluster Abell 1795 challenges the standard paradigm that giant radio haloes are exclusively the product of major cluster mergers.

Outlier Status: Abell 1795 is an outlier because it hosts a large, flat-spectrum halo in a dynamically relaxed system. This suggests that either sufficient turbulence persists in some relaxed clusters to sustain particle re-acceleration, or that hadronic interactions play a more significant role than previously thought.
Hadronic Origin: The close correspondence between the radio and X-ray profiles, combined with the flat spectral index, makes a hadronic origin (secondary electrons from proton-proton collisions) a compelling hypothesis for Abell 1795, though the authors note that modeling the required energy density scaling remains a challenge.
Observational Bias: The authors argue that large radio haloes in cool-core clusters may have been overlooked in past studies due to limited dynamic range near bright central AGN and the difficulty of detecting faint, extended emission in the presence of steep-spectrum filaments.
Methodological Advance: The study demonstrates the effectiveness of the facetselfcal pipeline for MeerKAT data, significantly improving image quality and enabling the detection of diffuse structures previously obscured by calibration artifacts.

In conclusion, the results indicate that the population of radio haloes may be more diverse than current models predict, with some relaxed clusters retaining the necessary conditions for large-scale non-thermal emission, potentially driven by hadronic processes or residual turbulence.

1. The "Ghost" in the Calm (Abell 1795)

2. The Two-Faced Cluster (Abell 1775)

3. The "Ghost Cavity" Connection

4. Why Did We Miss This Before?

The Bottom Line

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