Galaxy clusters in the early universe were thought to be relatively orderly systems, with turbulence, shocks and magnetic fields emerging gradually as they grew and merged. Observations from South Africa’s MeerKAT radio telescope now reveal that clusters were already turbulent and magnetised more than seven billion years ago.
In 2025, two major MeerKAT-based studies detected vast, faint clouds of radio emission, known as radio halos, both in extremely distant galaxy clusters and across a large population of nearer systems. Together, the results suggest that non-thermal activity driven by turbulence and magnetic fields formed much earlier and is far more common than existing models predicted.
Radio halos are enormous, typically between three and six million light-years wide. They are regions of diffuse radio emission, produced when galaxy clusters collide and merge. This injects turbulence into the hot gas between galaxies and re-accelerates charged particles that emit radio waves as they spiral through magnetic fields.
Until recently, radio halos had been detected only in low-redshift galaxy clusters (z ≲ 0.4). That changed when a team led by the University of the Witwatersrand in Johannesburg, South Africa, used MeerKAT to study1 six massive galaxy clusters at redshifts greater than one — systems whose light has been travelling for more than seven billion years.
“We detected radio halos filling the central regions of these distant clusters,” said Dakalo Phuravhathu, a PhD candidate at Wits University and lead author of one of the studies. “Before this, there were no confirmed detections of radio halos at these redshifts.”
All six clusters showed diffuse radio emission, with halos extending up to a million light-years across. Their signals appeared dimmer at higher radio frequencies, consistent with expectations that energetic particles in the early universe lost energy rapidly to intense background radiation.
Models had predicted that such energy losses would suppress radio emission at high redshift, leading to detection rates below 10%. The MeerKAT results challenge that assumption.
“These detections tell us that non-thermal processes like turbulence and magnetic field amplification were already active when galaxy clusters were still forming,” Phuravhathu said. “They are not late-stage features. They are fundamental from early on.”
The findings suggest that massive clusters were already injecting substantial turbulent energy into their environments at a much earlier stage of cosmic history than previously thought.
Mapping hidden radio structures
While the high-redshift detections reveal when cosmic turbulence switched on, a second large study shows just how widespread these processes are.
The MeerKAT Galaxy Cluster Legacy Survey (MGCLS)2,3 analysed 115 galaxy clusters at lower redshifts, initially observed during commissioning of the telescope. Instead of relatively quiet systems, researchers found a wealth of previously unseen radio structures.
“With MGCLS, we chart diffuse radio emission in unprecedented detail,” says Konstantinos Kolokythas, a postdoctoral research fellow jointly at Rhodes University in Makhanda, South Africa, and the South African Radio Astronomy Observatory. “We detected faint radio structures in more than half of the clusters we studied, and many of them had never been seen before.”
In total, the survey identified 103 diffuse radio structures spanning several distinct physical processes within galaxy clusters. Together with radio halos, these included relics — elongated features created by shockwaves during cluster mergers; mini-halos — smaller, diffuse emissions near cool cores; and phoenix sources — patches of aged, fossil plasma re-energised by compression or shocks.
Moving from individual detections to a statistically robust catalogue enables more stringent tests of models of particle acceleration and magnetic-field evolution. The MGCLS catalogue also revealed fine substructures within relics — filaments and tubes of electrons aligned with magnetic fields — providing new constraints for simulations of particle acceleration and cosmic magnetism.
Rewriting the timeline of cosmic weather
Taken together, the studies suggest a revised timeline for the evolution of galaxy clusters.
In the early universe, turbulence, shocks, and magnetic fields were already active as clusters assembled. Over billions of years, these processes continued to shape cluster ‘weather’, influencing galaxy evolution, energy transport, and the behaviour of cosmic rays.
MeerKAT’s sensitivity is central to this new picture. Many of these diffuse structures escaped earlier detection not because they were rare, but because previous instruments lacked the sensitivity to detect them.
“These results show that we were missing a large fraction of non-thermal activity in galaxy clusters,” Kolokythas said. “These discoveries are a preview of what the SKA will routinely detect. They provide a benchmark for how future surveys should be designed and interpreted.”
References
Phuravhathu, D., Sikhosana, S. & Hilton, M. et al. Mon. Not. R. Astron. Soc. 542, 1544–1561 (2025).
Kolokythas, K., Knowles, K. & Govoni, F. et al. Mon. Not. R. Astron. Soc. 543, 1638–1704 (2025).
Kolokythas, K., Sikhosana, S. & Hilton, M. et al. Preprint at arXiv https://doi.org/10.48550/arXiv.2509.05442 (2025).