Astronomers using South Africa’s MeerKAT radio telescope have directly detected faint neutral hydrogen gas across billions of light-years. This marks a critical breakthrough for hydrogen intensity mapping, allowing researchers to study the deep universe without relying on separate optical galaxy surveys.
Detecting the Deep Universe Without Optical Surveys
An international team of researchers spanning the University of the Western Cape and the University of Manchester utilized radio observations to capture the faint emission line of neutral hydrogen. Neutral atomic hydrogen naturally radiates at a wavelength of 21 centimetres known as the 21-cm line. As the universe expands, this signal stretches into longer wavelengths through a process called redshift, allowing scientists to look further back in time. The result, published in The Astrophysical Journal Letters, marks a significant step towards using neutral hydrogen to chart the Universe on the largest scales — and to probe the dark matter and dark energy that shape it.
While previous robust detections at such distances required pairing radio data with optical galaxy surveys, the new effort relied entirely on MeerKAT radio telescope observations. The researchers analyzed approximately 96 hours of data, capturing signals from two distinct epochs corresponding to redshifts of roughly 0.32 and 0.44. This means the emission travelled for roughly four to five billion years before reaching Earth, tracing hydrogen over scales of a few megaparsecs — a few times the distance between our Milky Way and its neighbouring galaxy, Andromeda.
Dr Sourabh Paul, lead author of the study, initiated the project in 2021 at the University of the Western Cape while he was a postdoctoral researcher in the group of Prof. Mario Santos.
Dr. Paul noted that 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.
Unexpected Insights from 2018 Commissioning Data
The data behind this discovery carry a twist. The observations were not originally designed for this experiment, demonstrating the exceptional sensitivity and versatility of the MeerKAT telescope, and were obtained in 2018 before MeerKAT had started science operations, according to the South African Radio Astronomy Observatory (SARAO) which built and operates MeerKAT in the Karoo region of the Northern Cape.
This was a challenging data analysis process, requiring a detailed understanding of the many sources of contamination that can affect such a faint measurement, Prof. Santos
We obtained these data to demonstrate the technical readiness of the telescope, not with any particular scientific goal in mind. Fernando Camilo
That preliminary dataset has since served multiple scientific purposes. Since then, however, this exquisite dataset has been used to investigate the star formation history of the universe, and now has also been used to demonstrate the power of the intensity mapping method to map the deep Universe,
Dr. Camilo explained.
Parallel Discoveries and Wide-Bandwidth Capabilities
The telescope’s broad frequency range continues to yield unexpected bonuses. Alongside neutral hydrogen measurements, MeerKAT’s wide bandwidth enabled the surprise detection of the most distant hydroxyl megamaser ever detected over eight billion light-years away. Operating like a natural space laser in a violently merging galaxy, the signal was boosted via gravitational lensing enough to detect it, allowing researchers to spot it in just five hours of observing time that typically requires hundreds of hours of observation given its distance and rarity.
This discovery was made possible by the sensitivity and wide frequency coverage of the MeerKAT radio telescope. Its ability to detect faint signals over a broad frequency range allows astronomers to search for spectral lines across large cosmic volumes. A spectral line is a cosmic chemical fingerprint, and detecting those frequencies tells astronomers what the gas is made of.
Implications for Next-Generation Global Facilities
These technical successes lay vital groundwork for forthcoming international mega-projects. The rapid detection suggests that future surveys with MeerKAT and the upcoming SKA Observatory could uncover many more such distant, extreme objects. Its ability to find this so quickly proves that we finally have the technology to see faint signals from the very distant past. It’s a preview of what the upcoming Square Kilometre Array (SKA), a unique, one-of-a-kind international mega-project, might achieve.
A highly complementary next-generation facility called the next-generation Very Large Array (ngVLA) is being planned and designed for construction in the US. The SKA Observatory (SKA-Low and SKA-Mid) focuses on low-to-mid radio frequencies, while the ngVLA will operate at much higher frequencies. Together, they will form two of the major pillars of next-generation global radio astronomy, giving astronomers a new way to study how galaxies evolved in the early universe.
