Astronomers have uncovered a vast cloud of energetic particles – a ‘mini halo’ – surrounding one of the most distant galaxy clusters ever observed, using the LOFAR radio telescope, marking a major step forward in understanding the hidden forces that shape the cosmos.
Published by the editorial team, 27 June 2025
The mini-halo is at a distance so great that it takes light 10 billion years to reach Earth, making it the most distant ever found – doubling the previous record. It demonstrates that entire galaxy clusters, among the largest structures in the Universe, have been immersed in high-energy particles for most of their existence.
Such a mini-halo consists of highly energetic charged particles in the vacuum between galaxies in a cluster, which together emanate radio waves which we can detect from Earth. This latest discovery shows that even in the early Universe, galaxy clusters were already shaped by energetic processes.
The international team of researchers behind the discovery was co-led by Dr Julie Hlavacek-Larrondo of the Université de Montréal in Canada and Dr Roland Timmerman of Durham University, UK. The researchers analysed data from the Low Frequency Array (LOFAR) radio telescope – a vast network of over 100,000 small antennas spanning eight European countries.
While studying a galaxy cluster named SpARCS1049, the researchers detected a faint, widespread radio signal. They found that it did not emanate from individual galaxies, but from a vast region of space filled with high-energy particles and magnetic fields.
This diffuse glow, stretching over a million light-years, is a telltale sign of a mini-halo – a structure astronomers have only been able to observe in the nearby Universe up until now. Dr Hlavacek-Larrondo said: “It’s as if we’ve discovered a vast cosmic ocean, where entire galaxy clusters are constantly immersed in high-energy particles.”
Dr Timmerman, of Durham University’s Centre for Extragalactic Astronomy, said: “It’s astonishing to find such a strong radio signal at this distance – it means these energetic particles and the processes creating them have been shaping galaxy clusters for nearly the entire history of the Universe.”
There are two likely explanations behind the formation of the mini-halo. One is supermassive black holes which lie at the hearts of galaxies within a cluster and can eject streams of high-energy particles into space. However, astronomers are still trying to understand how these particles would be able to migrate away from the black hole to create such a gigantic cloud of particles, while maintaining so much of their energy.
The second explanation is cosmic particle collisions. This is when charged particles within the hot plasma of the galaxy cluster collide at near-light speeds, smashing apart into the highly energetic particles we observe. This new discovery gives us a rare look at what galaxy clusters were like just after they formed.
It not only shows that galaxy clusters have been infused with these high-energy particles for billions of years more than previously known, but also allows astronomers to study where these high-energy particles come from.
It suggests that black holes and/or high-energy particle collisions have been enriching the environment of galaxy clusters much earlier than expected, keeping them energised over billions of years.
With upcoming telescopes like the Square Kilometer Array (SKA), scientists will be able to detect even fainter signals and further explore the role of magnetic fields, cosmic rays, and energetic processes in shaping the Universe.
Dr Hlavacek-Larrondo added: “We are just scratching the surface of how energetic the early Universe really was. “This discovery gives us a new window into how galaxy clusters grow and evolve, driven by both black holes and high-energy particle physics.”
The findings have been accepted for publication in Astrophysical Journal Letters, with the pre-print version of the paper now published.
