Astronomers have picked up an exceptionally faint radio signal from hydrogen gas billions of light-years away, using South Africa’s MeerKAT telescope.
The international team, led by researchers at the University of Manchester and the University of the Western Cape, said the detection shows a new way to map huge stretches of the Universe without identifying galaxies one by one.
The study, published in The Astrophysical Journal Letters, used MeerKAT to measure emissions from neutral hydrogen from a time when the Universe was several billion years younger than it is today.
Neutral hydrogen gives off a weak radio emission known as the 21-centimeter line. As the Universe expands, that signal stretches to longer wavelengths as it travels through space. By measuring that shift, astronomers can study hydrogen from different periods in cosmic history.
Instead of measuring individual galaxies, the method used here detects the combined radio glow from hydrogen in large numbers of galaxies that cannot be individually resolved. The researchers said that makes it possible to examine enormous volumes of space and build a three-dimensional picture of how matter is distributed.
Previous reliable measurements of hydrogen at these distances generally needed radio telescope observations to be combined with data from optical galaxy surveys. In this study, the team said it directly identified the hydrogen intensity mapping signal using MeerKAT radio observations alone.
Researchers analysed about 96 hours of MeerKAT observations and found the signal from two separate periods in cosmic history. The emissions travelled for about four to five billion years before reaching Earth.
The measurements let the team trace hydrogen across distances of several million light-years, roughly comparable to the separation between the Milky Way and the neighbouring Andromeda galaxy.
“This is a very exciting milestone,” lead author Dr Sourabh Paul said.
“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 researchers said extracting such a weak signal meant carefully accounting for sources of interference that could distort the measurements.
“This was a challenging data analysis process, requiring a detailed understanding of the many sources of contamination that can affect such a faint measurement,” Professor Santos said.
“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.”
Co-author Dr Zhaoting Chen said the findings open new opportunities to measure neutral hydrogen across cosmological distances and study how galaxies formed and changed over time.
“Neutral hydrogen is one of the key ingredients for understanding how galaxies form and evolve.
“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 team said hydrogen intensity mapping is expected to become an important scientific focus for the Square Kilometre Array Observatory, for which MeerKAT is a precursor telescope.
“MeerKAT continues to open new windows for cosmology. 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,” Professor Laura Wolz said.
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