Astronomers have made a groundbreaking discovery in the Abell 3558 galaxy cluster, revealing a peculiar mini-halo structure. This mini-halo, located in the cluster’s center, was uncovered through a detailed series of radio observations.
Abell 3558: A Massive And Complex Galaxy Cluster
Abell 3558, often referred to as A 3558, is a massive galaxy cluster situated about 700 million light-years away. With a redshift of approximately 0.047, this cluster is home to thousands of galaxies bound together by gravity. These clusters are among the largest known gravitationally-bound structures in the universe, and they provide an essential laboratory for astronomers to study galaxy evolution and cosmology.
This new research, conducted by an international team of astronomers led by Keegan Trehaeven from Rhodes University in South Africa, focused on the diffuse radio emissions found at the heart of Abell 3558. The team aimed to better understand the origin of these emissions, which had been previously detected but never fully explored. To this end, they used advanced radio telescopes, including the MeerKAT radio telescope and the Upgraded Giant Metrewave Radio Telescope (uGMRT), to observe the cluster in multiple bands of radio frequencies.
New Observations Reveal An Extended And Complex Emission
The study’s findings, published on the arXiv preprint server on July 10, 2025, revealed some fascinating details about the diffuse radio emission in Abell 3558. The emission was found to be more extensive than previously thought, stretching to a size of about 1.8 million light-years—one of the largest known radio emissions in any galaxy cluster.
The observations revealed that the emission has a radio power of 68 ZW/Hz at 1.4 GHz, a measure of the strength of the radio signals. This power level and the extent of the emission suggested that the phenomenon was much more intricate than initially believed. The astronomers also discovered a faint northern extension of the emission beyond the innermost cold front of the cluster.
What Powers This Peculiar Mini-halo?
The nature of the radio emission in Abell 3558 is a key part of the mystery. The team’s detailed study found that the emission exhibited a spectral index of 1.18, indicating varying levels of radio frequency across different regions of the cluster. A flatter spectrum was detected near the X-ray cold front, suggesting the presence of turbulent re-acceleration of the gas. This could be due to gas sloshing, a phenomenon that occurs when the gas within a galaxy cluster is disturbed by gravitational forces, creating turbulent motions.
Beyond the flatter region, the study found a steeper spectrum of emissions filling an X-ray cavity. This cavity is a high-temperature, low-pressure region within the cluster. The findings point to larger-scale sloshing turbulence that could be responsible for the extended and complex radio emission structure.
A Mini-halo Confined By Cold Fronts
Based on the observational data, the astronomers concluded that the diffuse radio emission in Abell 3558 forms a mini-halo-like structure. Mini-halos are typically found in galaxy clusters that are dynamically relaxed, where no major merger events have occurred. These halos are generally thought to be powered by turbulence caused by gas sloshing and possibly by active galactic nucleus (AGN) feedback processes.
In the case of Abell 3558, the radio emission appears to be confined by the cold fronts in the core of the galaxy cluster. This cold front, a region of the cluster where gas is cooler and denser, plays a crucial role in shaping the mini-halo. The alignment of the emission with a low-entropy gas trail suggests that the sloshing turbulence could be on a larger scale, further contributing to the formation of the mini-halo.
