Astronomers Detect Ancient Hydrogen Signal to Help Map the Universe

Astronomers have directly detected an extraordinarily faint radio signal from hydrogen gas billions of light-years away, using South Africa's MeerKAT radio telescope, in a step that could help build far larger three-dimensional maps of the universe. The international team, led by researchers at the University of Manchester and the University of the Western Cape, published the results in The Astrophysical Journal Letters.

The technique, called hydrogen intensity mapping, does not attempt to identify individual galaxies one at a time. Instead, it measures the combined radio glow produced by neutral hydrogen in large numbers of galaxies that cannot be resolved separately, using the gas's naturally weak 21-centimeter radio emission. Because the universe is expanding, this signal stretches to longer wavelengths as it travels, letting astronomers study hydrogen from different eras of cosmic history by measuring that shift.

Previous reliable measurements of hydrogen at these distances generally required combining radio telescope observations with data from optical galaxy surveys. In this study, the researchers directly identified the hydrogen intensity mapping signal using about 96 hours of MeerKAT observations alone, finding the signal from two separate periods in cosmic history. The emissions had traveled for approximately four to five billion years before reaching Earth, letting the team trace hydrogen across distances of several million light-years — roughly comparable to the separation between the Milky Way and the Andromeda galaxy.

"This is a very exciting milestone," said Dr. Sourabh Paul, the study's lead author. He said hydrogen intensity mapping has long been seen as a promising way to map the universe efficiently, but that the signal is extremely faint and difficult to isolate from foreground emission, human-made radio-frequency interference and instrumental effects. Professor Santos, a co-author, said the data used in the study were taken in 2018, when MeerKAT had only just started science operations, and that there is now a large amount of MeerKAT data still to be explored with this method.

Dr. Zhaoting Chen, another co-author, said neutral hydrogen is a key ingredient for understanding how galaxies form and evolve, and that intensity mapping lets researchers measure a collective signal across large cosmic volumes without detecting every individual galaxy. The researchers said hydrogen intensity mapping is likely to become an important focus for the upcoming Square Kilometre Array Observatory, for which MeerKAT serves as a precursor telescope.