The galaxy exhausted its fuel and then blocked the arrival of fresh fuel.
In the early Universe, barely three billion years after its birth, some of the most prolific stellar factories in existence suddenly fell silent — and stayed that way. Astronomers at the University of São Paulo, working with international collaborators, have traced this cosmic quieting to violent collisions between galaxies of comparable mass, events that simultaneously ignited and extinguished star formation by feeding central black holes whose energy then sealed off the very gas that sustained new stars. The discovery draws a direct evolutionary line between two populations long studied in isolation, suggesting that nearly every massive dead galaxy was once a furious, dust-shrouded engine of creation — and that the violence which made it great was also what killed it.
- Massive galaxies in the early Universe were dying young — shutting down star formation within a billion years of their peak activity — and no one fully understood why.
- Computer simulations revealed that 86 to 96 percent of these silent giants had previously blazed as dusty star-forming galaxies producing up to 500 new stars per year, linking two populations once thought unrelated.
- The trigger was catastrophic: head-on collisions between galaxies of similar mass drove cold gas to galactic cores, igniting extreme starbursts while simultaneously gorging the central supermassive black hole.
- The black hole's unleashed energy then heated surrounding gas halos, cutting off the fuel supply and quenching star formation in as little as 400 million years — a cosmic blink.
- James Webb observations are now finding far more of these early dead giants than models predicted, signaling that current simulations still undercount dusty galaxies and must be refined before the picture is complete.
Somewhere in the early Universe, when it was barely three or four billion years old, massive galaxies that should have been in their prime suddenly went quiet. They stopped making stars and stayed that way — cosmic graveyards drifting through the void while the Universe expanded around them. Astronomers have long puzzled over how these enormous systems assembled so much stellar mass so quickly after the Big Bang, and why they ceased star formation while still young, unlike our own Milky Way, which continues producing roughly one new solar mass of stars per year even now.
A team at the University of São Paulo, working with international collaborators, may have found the answer by connecting two seemingly unrelated galaxy populations. Dusty star-forming galaxies — shrouded in thick clouds that block visible light but blaze at infrared wavelengths — churn out up to 500 solar masses of new stars annually. Massive quiescent galaxies, by contrast, are silent giants packed with stars but making almost none. Using simulations of galaxies at redshifts between 2 and 4, the researchers found that between 86 and 96 percent of quiescent giants had previously existed as dusty star-forming systems. Nearly every silent giant had lived a violent, prolific youth.
The key difference for those that shut down early was a major merger — a violent collision with another galaxy of comparable mass. These crashes concentrated vast quantities of cold gas at galactic cores, triggering extreme bursts of star formation while simultaneously feeding the central supermassive black hole. For a brief period, the system became extraordinarily active. But the black hole's growth released enormous energy into the surrounding gas halo, heating it and preventing cool gas from falling inward to fuel new stars. The galaxy exhausted its supply and then blocked any replenishment. Simulations showed this quenching could occur within 400 to 900 million years of peak dusty activity. Galaxies that avoided early major mergers, by contrast, faded more gradually.
The work offers a coherent explanation for one of astronomy's most persistent puzzles, yet it also opens new ones. James Webb observations have uncovered far more massive quiescent galaxies in the early Universe than models predicted, and current simulations still underestimate the number of dusty star-forming systems. Closing that gap will require more sophisticated modeling and sharper observations — work that may fall to the Giant Magellan Telescope, currently under construction in Chile, whose 24.5-meter mirror promises images three to four times more detailed than Webb's, potentially illuminating how the first generations of massive galaxies were both built and silenced.
Somewhere in the early Universe, when it was barely three or four billion years old, something strange happened. Massive galaxies that should have been in their prime—still young, still hungry, still making stars by the hundreds each year—suddenly went quiet. They stopped. The stellar factories shut down. And they stayed dead, cosmic graveyards floating in the void while the Universe continued its expansion around them.
Astronomers have puzzled over this for years. How did these enormous galaxies assemble so much stellar mass so quickly after the Big Bang? And more mysteriously, why did they stop making stars while they were still young? Our own Milky Way, by contrast, formed early in cosmic history and has never fully quit—it still produces about one solar mass worth of new stars each year, even now, 13.5 billion years after its birth. These massive quiescent galaxies, as astronomers call them, followed a radically different script.
A team of researchers at the University of São Paulo, working with international collaborators, may have found the answer by connecting two seemingly unrelated populations of galaxies. On one side are the dusty star-forming galaxies, or DSFGs—some of the most extreme stellar factories ever observed. These systems churn out up to 500 solar masses of new stars annually, roughly 500 times the Milky Way's current rate. They're wrapped in thick clouds of dust that block visible light, making them invisible to traditional telescopes but brilliant at submillimeter and infrared wavelengths. The Atacama Large Millimeter Array and the James Webb Space Telescope have revealed thousands of these violent, prolific systems.
On the other side are the massive quiescent galaxies—the silent giants that appear later in cosmic history, packed with stars but making almost none. The researchers used computer simulations to trace the evolutionary histories of galaxies at redshifts between 2 and 4, corresponding to when the Universe was three to four billion years old. What they found was striking: between 86 and 96 percent of the massive quiescent galaxies had previously existed as dusty star-forming galaxies. Nearly every silent giant had lived a violent, prolific youth. But the reverse was not true—many dusty galaxies never became quiescent. They kept making stars for much longer before gradually slowing down. Something different happened to the ones that shut down early.
That something was a cosmic collision. The researchers found that the ancestors of massive quiescent galaxies typically experienced a major merger with another galaxy of similar size early in their histories. These were not gentle encounters. They were violent crashes involving enormous quantities of gas, dust, and stars. When two galaxies of comparable mass collided, the impact concentrated vast amounts of cold gas toward the center, triggering an extreme burst of star formation and simultaneously feeding the supermassive black hole at the galaxy's core. For a brief period, the system became extraordinarily active—stars forming at exceptional rates, the black hole consuming matter at high speed.
But the same process that ignited this activity ultimately extinguished it. As the central black hole grew, it released enormous amounts of energy into the surrounding environment. This energy heated the halo of gas surrounding the galaxy, preventing cool gas from falling back inward and being recycled into new stars. The galaxy exhausted its fuel and then blocked the arrival of fresh fuel. Without a supply of cool gas, star formation simply stopped. The simulations showed that galaxies following this path often quenched their star formation within less than one billion years after their peak dusty phase—somewhere between 400 and 900 million years, depending on the galaxy's age and redshift.
Meanwhile, dusty galaxies that avoided early major mergers followed a slower evolutionary path. They continued growing through long-term star formation and experienced major mergers much later, if at all. As a result, their star-forming activity faded more gradually rather than shutting down abruptly. The research offers a coherent explanation for one of astronomy's most persistent puzzles: how the Universe's earliest giant galaxies formed so rapidly and then stopped making stars while still young.
Yet the work has also revealed new questions. Recent observations from the James Webb Space Telescope have discovered far more massive quiescent galaxies in the early Universe than many models predicted. At the same time, Webb has provided new insights into dusty star-forming systems. The researchers acknowledge that existing models still underestimate the number of dusty galaxies observed in the real Universe. Closing this gap will require more sophisticated simulations and better observations. The Giant Magellan Telescope, currently under construction in Chile's Atacama Desert and expected to begin operations within the next decade, may provide the answers. With its 24.5-meter primary mirror, it will produce images three to four times more detailed than Webb, potentially revealing how galaxy mergers, starbursts, and black hole growth shaped the first generations of massive galaxies.
Notable Quotes
The merger of two galaxies concentrated large amounts of gas in the core, simultaneously triggering an extreme burst of star formation and intense feeding of the supermassive black hole.— Laerte Sodré Júnior, retired full professor at IAG-USP
The cold gas is rapidly consumed while the energy released by the active nucleus heats the surrounding halo gas and prevents it from cooling and being reincorporated into the galaxy, blocking the supply of raw material for new stars.— Laerte Sodré Júnior
The Hearth Conversation Another angle on the story
So these dusty galaxies are making hundreds of stars a year, and then suddenly they just stop. What actually kills that process?
A collision with another massive galaxy. When two of them merge, all that gas gets pushed to the center at once. It's like pouring fuel on a fire—you get this incredible burst of star formation and black hole growth. But the black hole's energy heats up all the surrounding gas and prevents it from cooling back down into the galaxy.
So the galaxy starves itself?
Exactly. It consumes its fuel in a frenzy, and then the energy it releases blocks any new fuel from arriving. Within a billion years, maybe much less, the star-making stops completely.
But not all dusty galaxies go through this. Some keep making stars for longer?
Right. The ones that avoid an early major merger with another giant galaxy keep growing more gradually. They experience collisions later, if at all. Their star formation fades slowly instead of shutting down abruptly.
And the evidence for this is in the simulations?
Yes, but also in the numbers. Between 86 and 96 percent of the massive dead galaxies they studied had previously been dusty star factories. The connection is unmistakable.
What's still missing?
The James Webb is finding more of these dead galaxies than the models predicted, and more dusty ones too. We're still underestimating how many exist. The next generation of telescopes will help us see what we're missing.