To build one Milky Way, you'd need up to 200,000 of them.
Some 800 million years after the Big Bang, the universe shed a primordial fog of neutral hydrogen and became the transparent cosmos we know — a transformation called reionization whose cause has long eluded astronomers. Now, the James Webb Space Telescope has identified 83 tiny, fiercely burning starburst galaxies from that pivotal era, suggesting that the humblest of cosmic actors may have lit the way for all the light that followed. Their smallness, it turns out, was not a limitation but a gift: low-mass galaxies let ultraviolet radiation escape more freely, and in sufficient numbers they may have done alone what larger, more conspicuous galaxies could not.
- For decades, the identity of the force that ended the universe's dark age has been one of cosmology's most contested open questions — and the stakes are nothing less than understanding how the observable cosmos became habitable to light itself.
- Webb's NIRCam instrument, trained on the gravitationally lensing Pandora's Cluster, detected 83 ancient starburst galaxies through a telltale green oxygen glow — a signal stretched into infrared by billions of years of cosmic expansion.
- The new analysis is ten times more sensitive than any previous effort, and it reveals these low-mass galaxies existed in numbers large enough, and burned with enough ultraviolet ferocity, to have ionized the universe's hydrogen fog entirely on their own.
- The math hinges on a modern comparison: similar 'green pea' galaxies today release about 25 percent of their ionizing light into surrounding space, and if their ancient counterparts did the same, the energy budget for reionization closes.
- The next measurement — pinning down exactly how much ultraviolet light these early galaxies actually released — will either cement this picture or force astronomers back to the drawing board.
About 800 million years after the Big Bang, the universe was still shrouded in a cold fog of neutral hydrogen that blocked light from traveling freely. Then something cleared it — a transformation astronomers call reionization. For decades, the culprit has been debated. New findings from the James Webb Space Telescope now point toward a surprising answer: tiny, barely visible galaxies that burned with extraordinary intensity.
A team led by Isak Wold of Catholic University of America and NASA's Goddard Space Flight Center identified 83 of these small starburst galaxies as they appeared when the universe was roughly six percent of its current age. Presented at the 246th meeting of the American Astronomical Society in Anchorage, the findings draw on data ten times more sensitive than anything previously applied to this question.
The galaxies were found within images from Webb's NIRCam as part of the UNCOVER program, which trained the telescope on Abell 2744 — a massive galaxy cluster nicknamed Pandora's Cluster whose gravity acts as a natural lens, amplifying light from objects far behind it. Wold's team searched for a specific signal: a green glow from doubly ionized oxygen, a marker of intense high-energy activity, stretched into infrared by the universe's expansion.
Their smallness may be precisely what made these galaxies so effective. Low-mass galaxies accumulate less surrounding hydrogen, giving ultraviolet light an easier escape route. Violent bursts of star formation compound this advantage, carving physical channels through interstellar gas. Wold's analysis shows these galaxies existed in sufficient numbers and radiated enough ultraviolet power to have driven reionization on their own.
The key comparison comes from the present day. Similar galaxies — sometimes called green peas — release roughly 25 percent of their ionizing light into surrounding space. If their ancient counterparts behaved the same way, the math closes: they could account for all the ultraviolet radiation needed to end the cosmic dark age. Rare now, such galaxies were abundant at the epoch when reionization was well underway. The next step is to measure more precisely how much light these ancient galaxies actually released — a result that could confirm or complicate the picture now coming into focus.
About 800 million years after the Big Bang, the universe was still wrapped in a thick fog — a vast, cold shroud of neutral hydrogen gas that blocked light from traveling freely through space. Then something cleared it. The fog lifted, the hydrogen was stripped of its electrons, and the cosmos became the transparent, star-filled expanse we inhabit today. Astronomers call this transformation reionization, and for decades they have argued about who did it. New findings from the James Webb Space Telescope are pointing toward a surprising answer: tiny galaxies, barely visible even to Webb, that burned with extraordinary ferocity.
A team led by Isak Wold, an assistant research scientist jointly affiliated with Catholic University of America and NASA's Goddard Space Flight Center, identified 83 of these small starburst galaxies as they appeared when the universe was roughly 800 million years old — about six percent of its current age of 13.8 billion years. Wold presented the findings at the 246th meeting of the American Astronomical Society in Anchorage, Alaska, drawing on data that is ten times more sensitive than anything previously brought to bear on this question.
The galaxies were found hiding in plain sight within images captured by Webb's NIRCam instrument as part of the UNCOVER observing program, led by Rachel Bezanson at the University of Pittsburgh. That program trained Webb on Abell 2744 — a massive galaxy cluster about four billion light-years away in the southern constellation Sculptor, nicknamed Pandora's cluster. The cluster's enormous gravitational mass acts as a natural lens, bending and amplifying light from objects far behind it, effectively extending Webb's already formidable reach deeper into the early universe.
To find the starburst galaxies, Wold and his Goddard colleagues Sangeeta Malhotra and James Rhoads searched for a very specific signal: a green glow produced by oxygen atoms that have been stripped of two electrons, a signature of intense, high-energy activity. That light, originally emitted as visible wavelengths in the ancient cosmos, was stretched into the infrared by the expansion of the universe over billions of years — precisely the kind of signal Webb's instruments were built to catch. Of the 83 galaxies flagged by this technique, 20 were selected for deeper examination using Webb's NIRSpec spectrograph.
The galaxies are almost incomprehensibly small. Malhotra put it plainly: to assemble the stellar mass of a single Milky Way, you would need somewhere between 2,000 and 200,000 of them. And yet their smallness may be exactly what made them so effective at reshaping the universe. Low-mass galaxies accumulate less neutral hydrogen in their surroundings, which means the ultraviolet light they generate has an easier path to escape. Starburst episodes compound this advantage — the violent rush of star formation not only produces enormous quantities of ultraviolet radiation but also carves physical channels through a galaxy's interstellar gas, giving that light additional routes to break free.
The question of what drove reionization has long been contested. The candidates have generally included large galaxies, small galaxies, and supermassive black holes at the centers of active galaxies. Recent work had begun tilting toward small galaxies, but the evidence was limited. Wold's analysis, with its tenfold improvement in sensitivity, now shows that these low-mass starburst galaxies existed in large enough numbers and radiated enough ultraviolet power to have done the job on their own.
The key comparison comes from the present day. Galaxies with similar properties that exist in the modern universe — a class sometimes called green peas — release roughly 25 percent of their ionizing ultraviolet light into the space around them. If the ancient starburst galaxies Wold's team identified behaved similarly, the math works out: they could account for the full quantity of ultraviolet radiation needed to ionize all of the universe's neutral hydrogen and end the cosmic dark age.
Such galaxies are rare now, making up only about one percent of galaxies in the local universe. But at redshift 7 — the cosmic epoch when reionization was well underway — they were abundant. Webb was designed in part to answer exactly this kind of question about the universe's formative era, and these results suggest it is delivering. The next step will be to pin down more precisely how much ultraviolet light these ancient galaxies actually released, a measurement that could either confirm or complicate the picture now coming into focus.
Citações Notáveis
When it comes to producing ultraviolet light, these small galaxies punch well above their weight.— Isak Wold, Catholic University of America and NASA's Goddard Space Flight Center
Starburst episodes not only produce plentiful ultraviolet light — they also carve channels into a galaxy's interstellar matter that help this light break out.— James Rhoads, NASA's Goddard Space Flight Center
A Conversa do Hearth Outra perspectiva sobre a história
So the universe was just... opaque for its first billion years?
Essentially, yes. Neutral hydrogen absorbed ultraviolet light before it could travel anywhere. The cosmos was foggy in a very literal sense.
And these tiny galaxies are what burned through the fog?
That's what the evidence is pointing toward. They were small, but they were burning stars at a furious rate, and that process generated enormous amounts of ionizing radiation.
Why would small galaxies be better at this than large ones?
Less surrounding gas to block the light. A massive galaxy tends to accumulate more neutral hydrogen around it, which traps the ultraviolet radiation before it can escape into the wider universe.
What's the gravitational lensing doing in this story?
Pandora's cluster is so massive it warps spacetime, bending light from objects behind it and magnifying them. It's like a natural telescope stacked on top of Webb itself.
How confident are they that these galaxies actually caused reionization?
The numbers are consistent — if these galaxies released UV light at the same rate as their modern analogs, they can account for all of it. But that's still a conditional. Confirming the escape fraction is the next hard measurement.
What are the modern analogs they keep referencing?
Green pea galaxies — compact, intensely star-forming galaxies we can observe nearby. They release about 25 percent of their ionizing light into space, which is unusually high.
Is it strange that the things that remade the universe are now almost gone?
It's one of the more haunting details. These galaxies made up a significant fraction of the early universe and then became nearly extinct. The cosmos they helped create is not the cosmos that favors them.