Why Massive Quiescent Galaxies in the Early Universe Stop Forming Stars

Massive quiescent galaxies are ancient relics that stopped forming stars far too early. Some of these giants, which emerged 3 or 4 billion years after the Big Bang, shut down their stellar factories within a single billion years of their birth. Astronomers can't explain why. But the Milky Way, now more than 13 billion years old and still quietly producing stars, makes the discrepancy glaring. Something violent must have intervened. Now a team of researchers thinks they've identified the culprit.

A Billion Years and Then Silence

They're not rare. These prematurely extinguished galaxies known as massive quiescent galaxies were once considered rare oddities, but different surveys returned different counts, leaving their true prevalence uncertain. But the JWST arrived and began finding far more of them than anyone expected, widening the tension between observation and theory. Powerful cosmological simulations like IllustrisTNG underpredicted their numbers by an order of magnitude, and that gap isn't a failure of the models exactly but a signal that something vital is missing from them.

But it doesn't take long. Their findings, published in Astronomy and Astrophysics, point toward a dramatic evolutionary sequence that turns prolific stellar nurseries into cosmic graveyards in under a billion years, and it's a process that we've only recently understood. Pablo Araya-Araya, a postdoctoral researcher at the Technical University of Denmark, led the investigation with collaborators from the University of São Paulo and institutions in the Netherlands and the United Kingdom.

The Dusty Counterparts

To understand why massive quiescent galaxies die young, the team looked at their apparent opposites, dusty star-forming galaxies or DSFGs, which are the most intense star factories in the early Universe, churning out up to 500 solar masses of fresh stars each year. They're dusty. But they cloak themselves in thick veils of dust that block optical light, so they blaze in infrared and submillimeter wavelengths where instruments like ALMA can see them clearly, while the Milky Way, by comparison, manages roughly one solar mass annually.

One is furiously alive. The other is conspicuously silent. At first glance, these two populations couldn't be more different. But models that successfully reproduce the observed number of DSFGs tend to underpredict massive quiescent galaxies, and models that reproduce MQs at high redshift tend to underpredict DSFGs. This reveals a persistent tension in galaxy formation models, the researchers explain. The physical recipes for extreme starbursts and for rapid quenching seemed contradictory.

When Galaxies Collide

The team deployed a new model on the Millennium simulation framework to trace the evolutionary paths of these populations. What emerged was a striking connection. Between 86 and 96 percent of massive quiescent galaxies at high redshift first passed through a DSFG phase. They were not separate species at all. One became the other.

"The merger of the 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," said Laerte Sodré Júnior, a retired full professor and doctoral advisor to the lead author. In that process, 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 and halting star formation in less than one billion years," explained Sodré.

The Missing Link

The sequence is brutal and efficient. A major merger between two galaxies drives gas into the central regions at catastrophic speeds. This triggers a starburst visible as a DSFG while simultaneously feeding the supermassive black hole lurking at the core. The resulting active galactic nucleus, or AGN, unleashes enough energy to heat the surrounding halo gas past the point where it can cool and fall back into the galaxy. Starvation sets in. The once-brilliant DSFG fades into a massive quiescent galaxy.

Feedback Destroys the Fuel

Mergers boost both. But the researchers explain that the rapid quenching of high-redshift MQs is driven by early mergers that result in overmassive SMBHs, so less AGN feedback energy is required to quench star formation in these systems. So it's the combined punch of supernova explosions and AGN feedback that researchers identified as the key mechanism, and the more massive the resulting black hole relative to the stellar mass, the less feedback energy is required to finish the job.

Not all galaxies follow this violent script. Most grow slowly. They're consuming their gas in measured fashion, with major mergers occurring much later when their impact is far less dramatic, but for the brightest DSFGs, the path to quiescence is swift and irreversible.

What the Model Misses

86 to 96 percent. That's how many of these quenched giants passed through a dust-enshrouded starburst first, according to the model, and the brightest DSFGs went on to become the most massive quiescent galaxies on the fastest timescales. So the model matched observations better than any previous effort, reconciling the DSFG and MQ populations in a single evolutionary framework.

Although the model has been successful, it still cannot reproduce the latest tally of massive quiescent galaxies coming from JWST, and as Sodré acknowledged, we're observing far more galaxies with submillimeter emissions than we predicted. But the discrepancy persists. Physics isn't accounted for.

What happens now is a familiar rhythm in astronomy. The model's shortcomings are not a dead end. They are guideposts. Future observations will refine the numbers. Simulations will absorb the new physics. The puzzle of how massive quiescent galaxies met their premature end is not fully solved, but the outlines of an answer are sharper than they were before. Mergers light the fuse, feedback snuffs the flame, and a brilliant DSFG becomes a silent relic in less than one billion years.

Key Findings at a Glance

• Between 86 and 96 percent of massive quiescent galaxies originally existed as dusty star-forming galaxies before rapid quenching shut down their star production.
• Major galaxy mergers concentrate gas, trigger extreme starbursts, and feed supermassive black holes, whose feedback heats halo gas and starves the galaxy of fresh fuel.
• The quenching process unfolds in less than one billion years, transforming some of the brightest objects in the early Universe into silent, dormant giants.

What Still Does Not Add Up

• JWST continues to find more massive quiescent galaxies than the model predicts, suggesting additional physical mechanisms remain unidentified.
• Submillimeter emissions from observed galaxies also exceed model predictions, pointing to gaps in our understanding of dust-obscured star formation.
• The IllustrisTNG simulation underpredicts MQ numbers by an order of magnitude, reinforcing that current cosmological models are incomplete at high redshift.