First published September 27, 2021.
Artist's impression of a galaxy in the early universe. Image: NRAO/AUI/NSF, S. Dagnello
In September 2021 a team led by Yoshinobu Fudamoto of Waseda University and the National Astronomical Observatory of Japan reported two galaxies that nobody had been looking for. Named REBELS-12-2 and REBELS-29-2, they existed more than 13 billion years ago, less than a billion years after the Big Bang, and they had escaped earlier surveys because thick dust hides almost all of their starlight. The radio telescope array ALMA picked them up by chance while observing other targets, and the find suggests that as many as one in five galaxies from that era may still be missing from astronomers' counts.
What was discovered
The two galaxies sit close on the sky to two brighter, already catalogued galaxies, REBELS-12 and REBELS-29, which were among about 40 distant targets in the Reionization Era Bright Emission Line Survey, or REBELS. While checking the data, Fudamoto noticed strong signals at positions slightly offset from the targets. Those signals turned out to come from separate galaxies at almost the same distance.
The newly found objects are often described as "neighbours" of the targets, and the follow-up check was revealing: the Hubble Space Telescope saw nothing at their positions in ultraviolet or visible light. They were invisible to the standard method for finding early galaxies.
How ALMA spotted what Hubble missed
The Atacama Large Millimeter/submillimeter Array is a set of 66 antennas on the Chajnantor plateau in northern Chile, about 5,000 metres above sea level. It is run by an international partnership led by the European Southern Observatory, the US National Radio Astronomy Observatory and the National Astronomical Observatory of Japan, and it observes at millimetre and submillimetre wavelengths.
Those wavelengths are the key. Dust absorbs the ultraviolet light of young stars and re-emits that energy as heat, which, after more than 13 billion years of cosmic expansion, reaches us as millimetre-wave radiation. ALMA detected two signatures from the hidden galaxies: the glow of the warm dust itself and a bright emission line from ionised carbon, known to astronomers as [CII]. The carbon line also let the team measure how far away the galaxies are.
Why hidden galaxies matter
Most of what we know about the first billion years of cosmic history comes from galaxies found in ultraviolet and visible light. If a significant share of early galaxies are wrapped in dust, that census is biased towards dust-free systems, and estimates of how fast stars formed in the young universe could be too low.
Fudamoto put it plainly when the paper appeared in Nature: "These new galaxies were missed not because they are extremely rare, but only because they are completely dust-obscured." Finding two such systems by accident, in a small patch of sky, suggests they could be fairly common. The team estimated that up to one in five galaxies of that era may have been overlooked so far.
There is a practical side to this. Models of how the first galaxies grew, how quickly heavy elements built up and how the universe's gas was reionised all depend on counting galaxies correctly. If dusty systems are routinely left out, those models start from an incomplete sample, and conclusions drawn from them about the pace of early star formation may need revising upwards.
The finding also raises a puzzle about timing. Dust is made by stars, especially in their final stages, so a galaxy that is already heavily dusty well under a billion years after the Big Bang must have built and recycled stars quickly.
A correction: not the edge of what we can ever see
Some coverage in 2021 claimed these galaxies are close to the furthest back in time humans could ever observe. That is not correct. The real limit for light is the cosmic microwave background, released about 380,000 years after the Big Bang, when the hot early universe became transparent. Before then, light was scattered endlessly by free electrons.
Galaxies have also been found much earlier than REBELS-12-2 and REBELS-29-2. The James Webb Space Telescope, launched at the end of 2021, has confirmed galaxies such as JADES-GS-z14-0, seen about 300 million years after the Big Bang, and in 2025 it reported MoM-z14, which is more distant still. The dusty REBELS galaxies remain important, but for what they reveal about hidden populations rather than as distance records.
What comes next
The study's authors argued that the only way to know how many dusty early galaxies exist is a blind, wide-area survey that does not rely on first finding a galaxy in ultraviolet light, and one that goes deeper than earlier plans to catch fainter, ordinary dusty galaxies. Pascal Oesch of the University of Geneva, a member of the team, said completing that census would be a main goal for surveys combining ALMA and the James Webb Space Telescope.
That combination is now working as hoped. Webb's infrared cameras can see through more dust than Hubble could, while ALMA confirms distances and measures gas and dust. Together they are steadily filling in the part of the early universe that was, until recently, behind a curtain.
Quick facts
- Names: REBELS-12-2 and REBELS-29-2
- Epoch: more than 13 billion years ago, under 1 billion years after the Big Bang
- Found with: ALMA, in data from the REBELS survey
- Published: Nature, 22 September 2021, "Normal, dust-obscured galaxies in the epoch of reionization", Fudamoto et al.
- Why they were hidden: dust absorbs their ultraviolet and visible light
Sources: National Astronomical Observatory of Japan press release, 23 September 2021; Vice, September 2021; Wikipedia: Atacama Large Millimeter Array; Wikipedia: Cosmic microwave background; Wikipedia: JADES-GS-z14-0.
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