The universe is not silent, even if much of what it says arrives as faint radio whispers rather than light. South Africa’s MeerKAT telescope has become one of the world’s finest listeners, revealing structures in deep space that were simply invisible to previous instruments. At the centre of this effort is the MeerKAT Galaxy Clusters Legacy Survey, a Rhodes-University-led programme examining the largest gravitational systems in the cosmos: galaxy clusters.
These clusters contain hundreds or thousands of galaxies, all immersed in a vast atmosphere of electrified gas. When clusters collide or merge, they leave behind diffuse radio structures - large, faint clouds of emission produced by fast-moving particles interacting with magnetic fields. These features act as tracers of cosmic upheaval. They show where shock waves have passed, where turbulence is stirred, and how magnetic fields thread through systems millions of light years wide.
The newly published MGCLS II catalogue, led by Rhodes University and SARAO researcher Dr Konstantinos Kolokythas, marks a significant advance. It charts diffuse radio emission in 115 galaxy clusters, detecting more than half of them and expanding the known population to 103 structures; 60 of which had never been seen before.
“This catalogue shows what happens when you pair South African engineering with South African scientific leadership,” Dr Kolokythas says. “MeerKAT has revealed structures that were simply out of reach for previous telescopes, and each one teaches us something about how galaxy clusters grow, collide and evolve.”
The value of this work extends far beyond astrophysics. It showcases a research environment in South Africa that contributes directly to global knowledge, strengthens the country’s scientific standing, and trains researchers who will lead future work with the Square Kilometre Array (SKA). It also demonstrates the tangible public value of long-term, curiosity-driven research: understanding the forces shaping galaxy clusters helps refine models of cosmic magnetism, large-scale structure and even the behaviour of matter under extreme conditions.
Diffuse radio emission is notoriously difficult to study. It is faint, spread out, and easily confused with unrelated signals from individual galaxies. MeerKAT’s sensitivity and image quality finally allow these structures to be separated, measured and understood with confidence. MGCLS II provides their sizes, shapes, brightness and locations, backed by a comprehensive review of what is known about each cluster from decades of X-ray and optical studies.
“We wanted to build something that the international community can use immediately,” Dr Kolokythas explains. “Every source in the catalogue has measured properties and high-quality images. It is a foundation for future work, not just a list of detections.”
The survey also highlights the importance of studying clusters across different wavelengths. X-ray images reveal where hot gas sits in the cluster’s core, while optical data show the distribution of galaxies. When these are combined with MeerKAT’s radio observations, a clearer picture emerges. Halos in the centre tend to coincide with the densest X-ray gas, signalling turbulence stirred up by major mergers. Relics on the outskirts often align with sharp edges in X-ray brightness, evidence of shock waves sweeping through the cluster.
This kind of multi-wavelength research is central to the future of global astronomy. As the SKA era approaches, MeerKAT is already defining the standards that future surveys will follow. The MGCLS work demonstrates what becomes possible when instruments are built not only to detect discrete objects but to reveal the diffuse, low-surface-brightness structures that carry the history of cosmic events.
“MGCLS gives us a glimpse of the discoveries that will become routine with the SKA,” Dr Kolokythas says. “If MeerKAT can uncover this much at 1.28 GHz, the full SKA will completely transform our understanding of how the largest structures in the universe form.”
The project also underscores the strength of Rhodes University’s research culture. Its radio astronomers lead international collaborations, shape the scientific agenda in cluster studies, and contribute directly to the training pipeline that will supply expertise for the SKA. Their work positions the institution as a contributor to global science while also advancing South Africa’s strategic investment in radio astronomy.
For Dr Kolokythas, the appeal is both scientific and human. “What excites me most is that radio waves, which we cannot see or feel, allow us to trace the life cycle of galaxy clusters,” he says. “In those faint signals, the universe records the story of collisions, shocks and magnetic fields on unimaginable scales.”
MGCLS II is not just a dataset; it is a benchmark. It gives the world an atlas of structures shaped by ancient cosmic events, made possible through South African infrastructure and scientific leadership. It is evidence that cutting-edge research can originate from places committed to serving the common good, and that Rhodes University’s work – quiet, rigorous and globally recognised – contributes meaningfully to how humanity understands the universe.
For more on the MGCLS, go to: http://mgcls.sarao.ac.za/