An international team of astronomers from the University of the Western Cape (UWC) and the University of Manchester has used South Africa’s MeerKAT radio telescope to directly detect the faint radio emission from neutral hydrogen gas across cosmic distances. The signal, observed from a time when the Universe was several billion years younger than it is today, provides an important demonstration of a technique known as hydrogen intensity mapping.

The finding, published in The Astrophysical Journal Letters, marks a significant step towards using neutral hydrogen to map the three-dimensional structure of the Universe at large scales.

Neutral hydrogen naturally emits a faint radio signal known as the 21-cm line. Because the signal is stretched to longer wavelengths as the Universe expands, astronomers can use it to trace hydrogen gas at different stages of cosmic history. Rather than detecting individual galaxies one by one, hydrogen intensity mapping measures the combined emission from many unresolved galaxies, making it a powerful method for surveying very large volumes of the Universe efficiently.

Until now, robust detections of this signal at these redshifts have typically relied on combining radio observations with optical galaxy surveys. In this new study, the team has directly detected the hydrogen intensity mapping signal using radio observations from MeerKAT alone. Remarkably, the observations were not originally designed for this specific experiment, demonstrating the exceptional sensitivity and versatility of the MeerKAT telescope.

The team analysed approximately 96 hours of MeerKAT observations and detected the signal from two periods in cosmic history, corresponding to redshifts of approximately 0.32 and 0.44. This means the emission has travelled for roughly four to five billion years before reaching Earth. The measurement traces hydrogen over megaparsec scales, comparable to the distance between the Milky Way and its neighbouring galaxy, Andromeda.

“This is a very exciting milestone,” said Dr Sourabh Paul, lead author of the study. “Hydrogen intensity mapping has long been seen as a promising way to map the Universe efficiently, but the signal is extremely faint and difficult to isolate from foreground emission, human-made radio-frequency interference, and instrumental effects. Detecting it directly with MeerKAT shows that this technique is becoming a practical tool for cosmology.”

The project was conceived and initiated in 2021 at UWC, while Dr Paul was a postdoctoral researcher in the research group of Prof Mario Santos. The work highlights both the scientific leadership developing around MeerKAT in South Africa and the exceptional capability of the South African telescope for cosmology.

“This was a challenging data analysis process, requiring a detailed understanding of the many sources of contamination that can affect such a faint measurement,” said Prof Santos. “This result opens a new window for measuring neutral hydrogen over cosmological distances. It is particularly remarkable that the data used in this study were taken in 2018, when MeerKAT had only just started science operations. There is now a rich trove of MeerKAT data waiting to be explored with this method.”

“Neutral hydrogen is one of the key ingredients for understanding how galaxies form and evolve,” said Dr Zhaoting Chen, co-author of the study. “With intensity mapping, we do not need to detect every individual galaxy. Instead, we can measure the collective signal from hydrogen across large cosmic volumes, giving us a new way to study both galaxy evolution and the underlying matter distribution of the Universe.”

The detection also has important implications for future cosmological surveys. Hydrogen intensity mapping is expected to become a major science driver for the Square Kilometre Array Observatory, for which MeerKAT is a precursor telescope. “MeerKAT continues to open new windows for cosmology,” said Prof Laura Wolz, co-author of the study from the University of Manchester. “The fact that this signal can be extracted from observations that were not originally designed for hydrogen intensity mapping is very encouraging. It shows the enormous scientific value of MeerKAT data and points the way to future observations with SKAO.”

The study delivers proof for using neutral hydrogen as a cosmic tracer across vast volumes of the Universe. Future observations with longer integration times, larger sky coverage and improved control of systematics will allow astronomers to map hydrogen with higher precision, revealing how galaxies form, how dark matter shapes the cosmic web, and how the Universe has changed over billions of years.

Paper: “A direct detection of neutral hydrogen intensity mapping on Mpc scales at z ≅ 0.32 and z ≅ 0.44”
Authors: Sourabh Paul, Zhaoting Chen, Mario G. Santos, and Laura Wolz
Journal: The Astrophysical Journal Letters, 2026, 1005:L56 https://doi.org/10.3847/2041-8213/ae808f