The earliest massive galaxies were already primed for intense star formation
In the universe's earliest chapter, when stars were just beginning to write themselves into existence, a galaxy known as REBELS-25 was already burning through vast reserves of cold molecular gas — fuel for star formation at a scale that defies easy imagination. Astronomers, peering back 13 billion years with radio telescopes in New Mexico and the Chilean Andes, have now directly observed that fuel for the first time, replacing long-held inference with evidence. The discovery affirms that the universe's first massive galaxies were not improvising their growth, but were already richly provisioned — and it opens a new era of inquiry into how the cosmos first learned to build itself.
- A fundamental cosmological question — how did early galaxies grow so massive so fast — has gone unanswered for decades, sustained only by indirect guesswork.
- Detecting cold molecular gas at a redshift of 7.3 is extraordinarily difficult: the ancient glow of the Big Bang itself acts as cosmic interference, growing louder the further back in time astronomers look.
- A Leiden University team combined the NSF's Very Large Array and ALMA to cut through that noise, capturing the faintest carbon monoxide signal ever detected at such distances — the most remote low-energy CO detection in history.
- The signal confirmed that REBELS-25 was not barely surviving on scarce resources, but was already loaded with the raw material to forge stars at a furious, sustained rate.
- The discovery reframes cosmic dawn: rapid mass assembly in the universe's first billion years was not a fluke, but a natural consequence of galaxies that were already fuel-rich.
- The planned Next-Generation VLA will conduct these observations ten times faster, transforming REBELS-25 from a singular case study into the first data point of a sweeping new map of early galactic growth.
When the universe was barely 700 million years old — roughly five percent of its current age — a galaxy called REBELS-25 was already forging stars at a furious pace. Astronomers have now done something no one had managed before: they directly observed the cold molecular gas that powered it, reaching back to a time when the cosmos was still in its infancy.
The question driving the search was one that had long unsettled cosmologists. Early massive galaxies clearly grew with remarkable speed, but the fuel behind that growth had never been seen directly — only inferred, the way one might guess at a room's contents by watching what passes through its door. A team led by Ph.D. student Karin Cescon at Leiden University set out to open that door.
Using the National Science Foundation's Very Large Array in New Mexico and the Atacama Large Millimeter Array in Chile, the team hunted for carbon monoxide emissions — a reliable chemical signature of cold gas in space. The challenge was immense: the ancient light left over from the Big Bang grows brighter and more disruptive the further back in time one looks, and at REBELS-25's redshift of 7.3, it threatened to drown out any signal entirely. The team overcame it, detecting the most distant low-energy CO emission ever recorded.
What the signal revealed was not a galaxy scraping by on scarce resources, but one already primed with abundant fuel for intense star formation. The finding suggests that rapid mass assembly in the universe's first billion years was not a cosmic accident — it was made possible by galaxies that arrived at the task already well-supplied.
The discovery also points toward what comes next. The planned Next-Generation Very Large Array will make such measurements ten times faster, allowing astronomers to survey many more early galaxies and map, in earnest, how the first structures in the universe gathered their fuel and grew. REBELS-25 is, for now, a singular bright example — but the instruments to find what surrounds it are already taking shape.
When the universe was barely half a billion years old, a galaxy called REBELS-25 was already burning through fuel at a furious pace. Astronomers have now detected the cold molecular gas that powered this early star-making machine—the first direct observation of such fuel at these cosmic distances, reaching back to a time when the universe itself was only about 700 million years old, or roughly 5 percent of its current age.
The discovery matters because it answers a question that has long nagged at cosmologists: How did the earliest galaxies grow so massive so quickly? For years, astronomers suspected that bright, massive galaxies in the young universe must have had enormous supplies of gas available for star formation. But suspicion is not the same as proof. Until now, no one had actually seen the cold molecular gas itself at these distances. They could only infer its presence indirectly, the way you might guess at the contents of a locked room by watching what comes out of it.
A team led from Leiden University used two of the world's most powerful radio telescopes to change that. The National Science Foundation's Very Large Array, a sprawling antenna farm in Socorro County, New Mexico, searched for faint radio signals from carbon monoxide molecules—a reliable tracer of cold gas in space. The Atacama Large Millimeter Array, perched high in the Chilean Andes, provided complementary data. Together, they detected emission from a specific carbon monoxide line in REBELS-25, the most distant low-energy detection of this kind ever made. The signal was faint, but unmistakable. It told a clear story: this young galaxy was already loaded with the raw material needed to forge stars at a prodigious rate.
The technical challenge was formidable. The cosmic microwave background—the ancient light left over from the Big Bang itself—acts as a kind of cosmic noise, drowning out faint signals from the early universe. This interference grows worse the farther back in time you look. At the distance of REBELS-25, with a redshift of 7.3, the background radiation becomes significantly brighter, making it exponentially harder to detect cold gas emission. Karin Cescon, the Leiden University Ph.D. student who led the analysis, and her colleagues had to overcome this fundamental observational hurdle. They did, by using the right combination of instruments and techniques to cut through the noise.
What they found reshapes our understanding of cosmic dawn. REBELS-25 was not an anomaly scraping by with just enough gas to survive. It was already primed for intense star formation, its reservoir of cold molecular gas suggesting that some of the universe's earliest massive galaxies were built on foundations of abundant fuel. This is not a minor detail. It means that rapid mass assembly in the first billion years after the Big Bang was not a fluke or a mystery—it was enabled by the simple fact that these galaxies had the material they needed, already in place.
The discovery is also a proof of concept for what comes next. The Next-Generation Very Large Array, a planned observatory that will deploy antennas across New Mexico, West Texas, Arizona, northern Mexico, and beyond, will be able to make these measurements ten times faster. Where REBELS-25 is currently a singular bright case study, the ngVLA will enable astronomers to survey many more early galaxies, including fainter and more distant ones. Paired with ALMA, it will map how the first galaxies gathered their fuel and grew during the universe's infancy. For now, REBELS-25 remains the tip of the iceberg. But the tools to explore what lies beneath are already being built.
Notable Quotes
With these deep NSF VLA observations, we were able to overcome the observational challenges posed by the CMB.— Karin Cescon, Leiden University
The ngVLA will allow us to find and study cool gas in many more young galaxies, including those at even earlier times.— Jacqueline Hodge, Leiden University
The Hearth Conversation Another angle on the story
Why does it matter that we can see the actual gas, rather than just inferring it was there?
Because inference is educated guessing. When you see the gas directly, you can measure its mass, its temperature, its density—the actual physical conditions. You move from "this galaxy probably had a lot of gas" to "this galaxy had exactly this much gas, and here's what it tells us about how fast it could form stars."
And the cosmic microwave background was making this harder to detect?
Much harder. The CMB is ancient light from the Big Bang, and it fills all of space. When you're looking back 13 billion years, that background radiation is so bright it drowns out the faint signals from cold gas. It's like trying to see a candle in daylight.
So how did they manage to detect it at all?
They used the right telescopes—the VLA and ALMA together—and they knew exactly what frequency to look for. Carbon monoxide emits at specific wavelengths. If you tune your instrument to listen at those frequencies, you can pick out the signal even when there's noise around it.
What does this tell us about how galaxies actually form?
That the earliest massive galaxies didn't have to scrape by on scraps. They had fuel already waiting. That changes the question from "how did they manage to grow so fast?" to "what stopped them from growing even faster?"
And the next-generation telescope will do this better?
Ten times faster, for starters. But more importantly, it will let us study not just the brightest outliers like REBELS-25, but a whole population of early galaxies. We'll see the pattern, not just the exception.