Chandra X-ray image of SPT-CLJ2337−5942 in the 0.5 – 2.0 keV band, smoothed to 12 arcsec. The Gaussian smoothing kernel FWHM is indicated in the bottom-left corner. The radio halo emission is shown as white contours. Credit: arXiv (2025). DOI: 10.48550/arxiv.2509.08062
Using the MeerKAT radio telescope, an international team of astronomers has performed observations of a galaxy cluster designated SPT-CLJ2337−5942. The observational campaign revealed the presence of an ultra-steep spectrum radio halo in this cluster. The finding is presented in a paper published Sept. 9 on the arXiv preprint server.
Radio halos are enormous regions of diffuse radio emission, usually found at the centers of massive galaxy clusters. They showcase a regular morphology that tends to trace the turbulent X-ray emitting intracluster medium (ICM).
In general, radio halos have very low surface brightness, particularly at GHz frequencies, which makes them hard to detect. Their brightness increases at lower frequencies, unveiling the presence of these regions.
Some of them are ultra-steep spectrum radio halos (USSRHs), which can only be produced through turbulent re-acceleration. Therefore, finding new USSRHs could shed more light on the efficiency of particle re-acceleration and the role of cluster mergers.
Now, a group of astronomers led by Isaac S. Magolego of the University of the Witwatersrand in Johannesburg, South Africa, reports the detection of a new radio halo of this type. The new USSRH was identified in the galaxy cluster SPT-CLJ2337−5942, as part of the MeerKAT-South Pole Telescope survey.
“We identified the diffuse source through a systematic manual search for extended radio emission in the sample of 89 SPTpol galaxy clusters that lie within the MeerKAT-SPT 100 deg2 field,” the researchers wrote in the paper.
The MeerKAT UHF-band image revealed extended diffuse emission with a complex morphology, detected with high confidence at the center of SPT-CLJ2337−5942. The largest linear size of this emission was measured to be about 2.6 million light years at 816 MHz.
According to the study, the newfound radio halo has an integrated spectral index of 1.76 (at 578–986 MHz), therefore significantly steeper than that predicted by hadronic models, which points to an ongoing particle re-acceleration. The radio power of the halo at 1.4 GHz was measured to be 960 ZW/Hz, hence relatively low when compared to other radio halos.
By analyzing the data, the astronomers also found a striking spatial correlation between the filamentary-like diffuse radio emission SPT-CLJ2337−5942 and the X-ray brightness. This suggests a direct link between thermal ICM electrons, non-thermal electrons, and cluster magnetic fields.
Summing up the results, the authors of the paper underlined that the radio halo in SPT-CLJ2337−5942 is the highest redshift USSRH so far discovered and therefore could be crucial in advancing our knowledge about radio halo formation models. They hope that future observations with the Square Kilometer Array (SKA), together with ongoing low-frequency surveys such as MeerKAT-SPT, will bring more detections of high-redshift USSRHs.
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More information:
Isaac S. Magolego et al, Discovery of a z ~ 0.8 Ultra Steep Spectrum Radio Halo in the MeerKAT-South Pole Telescope Survey, arXiv (2025). DOI: 10.48550/arxiv.2509.08062
Journal information:
arXiv
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New radio halo discovered with MeerKAT (2025, September 17)
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