A team of astronomers has confirmed the presence of a giant radio halo extending across more than 3.3 million light-years inside the galaxy cluster RXCJ0232–4420. Using observations from the upgraded Giant Metrewave Radio Telescope (uGMRT) and South Africa’s MeerKAT telescope, researchers identified diffuse radio emission on a scale rarely associated with a relatively calm cluster environment.
The findings, published April 29 on the arXiv preprint server, strengthen the idea that large radio halos can exist even in systems that still preserve a cool core. According to the study led by Pralay Biswas from the National Center for Radio Astrophysics in Pune, the cluster displays characteristics of an intermediate dynamical state.
RXCJ0232–4420 was discovered in 2002 and had already drawn attention because earlier radio observations produced conflicting interpretations. Some studies described a compact mini-halo centered on one of the cluster’s brightest galaxies, while others suggested that the diffuse emission stretched across much larger distances.
Researchers Identify a Huge Structure Across the Full Cluster
The new observations confirmed that the diffuse radio emission extends beyond 3.3 million light-years at every observed frequency. According to the research paper, this establishes the existence of a genuine giant radio halo within RXCJ0232–4420. Astronomers also identified an eastern radio relic with an estimated linear size of around 980,000 light-years. The cluster itself contains two brightest cluster galaxies, known as BCG-A and BCG-B, separated by roughly 330,000 light-years.
Giant radio halo revealed in multiwavelength image of RXCJ0232–4420. Credit: arXiv
Previous observations had detected diffuse emission primarily around BCG-A, leading researchers to compare the source to a radio mini-halo. The broader structures revealed by uGMRT and MeerKAT changed that picture substantially.
Spectral Analysis Points to a Highly Uniform Halo
The study reports a spectral index of −1.17 for the radio halo and −0.85 for the eastern relic. These measurements help astronomers examine how radio emission changes with frequency across the cluster. Researchers found that the e-folding radius of the halo’s radio profile did not vary significantly with frequency. The study also found no signs of radial spectral steepening at different frequencies
uGMRT and MeerKAT observations of RXCJ0232–4420 revealing the cluster’s giant radio halo and eastern radio relic. Credit: Researchgate
The spectral maps showed very little change from one region to another, with most values ranging between about −1.0 and −1.3. As explained in the paper, these numbers are not especially steep for a radio halo linked to a cool-core cluster. The findings suggest that charged particles are likely being re-energized across much of the cluster, rather than only in a few isolated spots.
X-Ray Analysis Points to a Gently Disturbed Cluster
The team also examined how the cluster’s radio and X-ray emissions were connected. Their analysis revealed a strong positive link between the non-thermal radio emission and the hot X-ray-emitting gas spread throughout the cluster.
Further X-ray and thermodynamic measurements suggested that the cluster is in an intermediate dynamical state. Although RXCJ0232–4420 still contains a cool core, the observations also uncovered signs of mild structural disturbance. That makes the system somewhat unusual, since giant radio halos are typically associated with clusters undergoing far more violent mergers.