Young galaxies should be making stars. Finding them asleep was genuinely surprising.
In the earliest chapter of cosmic history, the James Webb Space Telescope has found galaxies that fell silent — pausing their star-making in an infant universe long assumed to be ceaselessly creative. A team led by doctoral student Alba Covelo Paz at the University of Geneva identified fourteen such dormant galaxies within the first billion years after the Big Bang, spanning an unexpectedly wide range of sizes. The discovery suggests that galaxy evolution has always been rhythmic rather than relentless — a universe that breathes in and out, not one that only exhales.
- Fourteen galaxies in the early universe have been caught in a state of suspended creation, defying the long-held assumption that young galaxies would be furiously forming stars.
- These dormant galaxies span an enormous range of masses — from 40 million to 30 billion solar masses — meaning no galaxy, large or small, is immune to these quiet phases.
- The likely culprit is stellar feedback: supernovas and stellar winds heating and expelling gas, silencing star formation for roughly 10 to 25 million years before conditions may reset.
- The critical uncertainty is whether these galaxies will wake again or remain permanently quenched — a question that current observations cannot yet resolve.
- A dedicated Webb program called 'Sleeping Beauties' is being launched to measure how long dormancy lasts and whether these cosmic pauses are part of a recurring cycle or a final ending.
The James Webb Space Telescope has found something unexpected in the early universe: galaxies that stopped making stars. Led by doctoral student Alba Covelo Paz at the University of Geneva, an international team identified fourteen dormant galaxies within the first billion years after the Big Bang — a finding that challenges decades of assumptions about how young galaxies behave.
By analyzing light from roughly 1,600 early galaxies using Webb's spectroscopic instruments, the team found clear signatures of galaxies gone quiet. What makes the discovery especially striking is the range of sizes involved — from 40 million to 30 billion solar masses — suggesting dormancy is not a quirk of extreme galaxies but a widespread phenomenon. Before this work, only four dormant early-universe galaxies had ever been found, all at the fringes of the size spectrum.
The most likely explanation is stellar feedback: when massive stars explode as supernovas or unleash powerful winds, they heat surrounding gas and push it outward, temporarily halting star formation. Paz estimates these galaxies went quiet between 10 and 25 million years before Webb observed them — a brief cosmic pause. If the expelled gas cools and falls back, star formation can resume, making galaxy evolution a cyclical, bursty process rather than a steady one.
The open question is whether these galaxies will restart or remain permanently silent. A dormancy lasting 50 million years or more would point to a different, more final mechanism at work. Webb's technological edge over Hubble — particularly its ability to detect redshifted infrared light and analyze stellar populations in fine detail — made these identifications possible where earlier telescopes could not.
To pursue the deeper questions this discovery raises, astronomers have planned a dedicated Webb program called 'Sleeping Beauties,' aimed at mapping the duration and mechanics of dormancy across cosmic time. For now, the finding reshapes how we understand the universe's first galaxies — not as relentless engines of creation, but as living systems that rest, reset, and begin again.
The James Webb Space Telescope has caught something astronomers did not expect to find in the infant universe: galaxies that have simply stopped making stars. An international team of researchers, led by doctoral student Alba Covelo Paz at the University of Geneva, identified fourteen of these dormant galaxies within the first billion years after the Big Bang—a discovery that upends long-held assumptions about how the earliest galaxies should behave.
For decades, astronomers assumed that young galaxies would be feverishly producing new stars. The universe was young, the thinking went, and these stellar nurseries should be working at full capacity. But the Webb telescope's sensitive spectroscopic instruments revealed something different. By examining the light from roughly 1,600 galaxies in the early universe, Paz's team found clear signatures of galaxies that had gone quiet—places where star formation had simply paused. The fourteen dormant galaxies they identified ranged enormously in size, from about 40 million solar masses to 30 billion solar masses, suggesting that this dormancy is not confined to galaxies of any particular size.
What makes this discovery significant is not just that these sleeping galaxies exist, but that they exist across such a wide spectrum of masses. Before this work, astronomers had spotted only four dormant galaxies in the early universe, and those four fell into extreme categories: either very small or very large. The new findings, uploaded to the preprint database arXiv in late June and not yet peer-reviewed, show that dormancy is a more common phenomenon than previously thought, affecting galaxies of all sizes. This shifts the conversation about how galaxies evolve from their birth to maturity.
The question then becomes: why do these galaxies stop forming stars? Several mechanisms could be at work. Supermassive black holes at the centers of galaxies emit intense radiation that can heat and strip away the cold gas necessary for star formation, potentially silencing a galaxy for good. Neighboring galaxies can also steal this cold gas or heat it beyond usefulness. But the evidence from these fourteen dormant galaxies points to something more temporary: stellar feedback. When massive stars explode as supernovas or unleash intense stellar winds, they heat the surrounding gas and blow it outward, creating a temporary lull in star formation. Paz explained that these galaxies appear to have gone quiet between 10 and 25 million years before the Webb telescope observed them—a relatively brief cosmic nap.
This short dormancy period is crucial to understanding what happens next. If the expelled gas falls back into the galaxy and cools again, star formation can resume. Paz describes this as a cyclical, bursty process: galaxies form stars intensely, then quiet down, then start again. The fourteen galaxies discovered fit this pattern, suggesting they are not dead but merely resting. However, uncertainty remains. If these galaxies stay dormant for another 50 million years or longer, it would suggest a different mechanism is at work—one that permanently quenches star formation rather than temporarily pausing it. Without longer observations, astronomers cannot yet be certain which scenario is unfolding.
What makes this discovery possible is Webb's technological advantage over its predecessor, the Hubble Space Telescope. Hubble had observed some of these galaxies before, but could not determine whether they were dormant. Webb's NIRSpec instrument can detect light that has been redshifted into the near-infrared spectrum and analyze its detailed properties, revealing the presence or absence of young stars. This spectroscopic precision is what allowed Paz and her team to identify the telltale signatures of middle-aged or old stars in galaxies that had stopped forming new ones.
The findings raise as many questions as they answer. How common is this dormancy in the early universe? How long do these quiet phases typically last? What determines whether a galaxy will restart or remain permanently quenched? To pursue these questions, astronomers have planned a dedicated Webb program called "Sleeping Beauties," which will focus specifically on discovering and studying dormant galaxies in the early universe. This program aims to measure how long galaxies remain in their quiet phase and to map out the mechanics of bursty star formation across cosmic time. For now, Paz and her colleagues have moved one step closer to understanding how the universe's first galaxies grew and changed. The rest of the story is still being written.
Citas Notables
The first discovery of a dormant galaxy in the early universe was such a shock because that galaxy had been observed before with Hubble, but we could not know it was dormant until JWST.— Alba Covelo Paz, University of Geneva
We now found 14 sources supporting this burstiness process, and we found that all of them have halted star formation between 10 and 25 million years before we observed them.— Alba Covelo Paz
La Conversación del Hearth Otra perspectiva de la historia
Why would anyone expect young galaxies to be dormant? Shouldn't they be making stars constantly?
That's exactly what astronomers thought. The early universe was supposed to be a time of vigorous activity—galaxies forming stars rapidly because they had all the raw materials and all the time ahead of them. Finding galaxies that had simply paused was genuinely surprising.
So what stops them? Is it permanent, or do they wake up?
That's the crucial question. The evidence suggests these particular galaxies are just sleeping, not dead. Stellar explosions and winds blow the gas away temporarily, but that gas can fall back and cool. If it does, star formation starts again. It's cyclical—bursts of activity followed by quiet periods.
How do you even know a galaxy is dormant? They're so far away.
The Webb telescope can split the light from these galaxies into its component wavelengths and read the spectrum like a fingerprint. If you see old stars and no young ones, you know star formation stopped. Hubble couldn't do that. It's a technological leap.
These fourteen galaxies—are they all the same, or completely different from each other?
Completely different. Some are as small as 40 million suns, others as massive as 30 billion. That's the real surprise. Dormancy isn't just something that happens to extreme galaxies. It happens everywhere, across the whole range of sizes.
How long do they stay asleep?
These ones have been quiet for 10 to 25 million years. That sounds long, but in cosmic terms it's brief. If they stay dormant much longer—say, 50 million years—it would mean something else is keeping them quiet, something more permanent. We won't know until we watch longer.
What comes next for this research?
There's a new Webb program starting called "Sleeping Beauties" dedicated to finding more of these dormant galaxies. The goal is to figure out how long they typically stay quiet and whether they all follow the same cycle. That will tell us whether this bursty pattern is how galaxies normally grow.