Composite image of the Abell 3558 environment. Credit: arXiv (2025). DOI: 10.48550/arxiv.2507.07549
An international team of astronomers has performed multi-band radio observations of diffuse radio emission in a galaxy cluster known as Abell 3558. As a result, the observational campaign detected that the cluster hosts a peculiar mini-halo. The finding was detailed in a paper published July 10 on the arXiv preprint server .
The largest known gravitationally-bound structures in the universe are galaxy clusters, which contain up to thousands of galaxies bound together by gravity. They form through accretion of mass and infall of smaller sub-structures and therefore could serve as excellent laboratories for studying galaxy evolution and cosmology.
Abell 3558, or A 3558 for short, is a massive galaxy cluster at a redshift of approximately 0.047. It has a dynamical mass of about 1.48 quadrillion solar masses and a bolometric X-ray luminosity at a level of 700 tredecillion erg/s.
Previous observations of Abell 3558 have found that its intracluster medium showcases mixed characteristics and detected a significant large-scale diffuse radio emission in the cluster center. A group of astronomers led by Keegan Trehaeven of the Rhodes University in South Africa decided to take a closer look at this emission, with the aim of disentangling its origin. For this purpose, they employed the MeerKAT radio telescope and the Upgraded Giant Metrewave Radio Telescope (uGMRT).
“Here we present a detailed study of the diffuse radio emission at the center of A 3558 using deep observations carried out with MeerKAT UHFband (816 MHz) and uGMRT Band3 (400 MHz), complemented by the MeerKAT L-band (1,283 MHz) and ASKAP (887 MHz) data,” the researchers wrote in the paper.
The observations conducted by Trehaeven’s team found that the diffuse radio emission in the center of Abell 3558 has a radio power of 68 ZW/Hz at 1.4 GHz and is more extended than previously thought. They identified a faint northern extension of this emission beyond the innermost cold front. This finding increases the projected largest linear size of the radio emission to about 1.8 million light years.
The study found that the investigated emission has an integrated spectral index of 1.18. The spectral index map shows a region of flatter spectrum just inside the X-ray cold front, which suggests a local turbulent re-acceleration. Beyond that, a region of steeper spectrum is observed, filling an X-ray cavity of higher temperature and entropy but with lower pressure.
According to the paper, the diffuse emission in Abell 3558 exhibits a mini-halo-like structure confined by cold fronts in the core, likely powered by turbulence from gas sloshing. Moreover, it showcases a ridge-like structure extending along the cluster axis aligned with a low-entropy gas trail, which may be powered by larger-scale sloshing turbulence.
Therefore, based on the obtained results, the astronomers concluded that the observed diffuse radio emission is a radio mini-halo.
In general, radio mini-halos are found mainly in dynamically relaxed systems hosting cool-cores, where no major merger event has occurred. Although their origin is still uncertain, it is assumed that they are powered by turbulent re-acceleration induced by gas sloshing and/or active galactic nucleus (AGN) feedback processes.
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More information:
K. S. Trehaeven et al, The Peculiar Mini-Halo in the Shapley Supercluster Member Abell 3558, arXiv (2025). DOI: 10.48550/arxiv.2507.07549
Journal information:
arXiv
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Galaxy cluster Abell 3558 has a peculiar mini-halo, observations suggest (2025, July 17)
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