How quasars shut down star formation in the early universe

Supermassive black holes lurk at the centers of massive galaxies, including our own Milky Way. Puzzlingly, supermassive black holes more than a billion times the mass of the sun appear to exist just a few hundred million years after the Big Bang, when the universe was less than 5% of its current age. As interstellar gas spirals towards such black holes, it accelerates to extreme speeds, heats up, and emits intense radiation across the electromagnetic spectrum, creating a “quasar.”

Theoretical predictions put to the test

Theoretical models of early galaxy evolution have long predicted that the energetic output of quasars should be sufficient to expel large masses of interstellar gas from their host galaxies at enormous speeds, exceeding several thousand kilometers per second. These “galactic winds”—enormous flows of gas driven out of galaxies—are thought to profoundly change galaxy evolution, but their detection in the first quasars has remained contentious.

Using observations from the James Webb Space Telescope, an international team led by researchers in Arizona and including theorist Tiago Costa of Newcastle University has now found clear signatures of galactic winds in a sample of quasars in the first billion years of the universe. Moreover, these winds appear to be more common and their speeds comparable, or even faster, than in quasars lighting up the universe at later times.

Tiago Costa, of Newcastle University’s School of Mathematics, Statistics and Physics, previously led theoretical studies predicting that powerful winds should be ubiquitous in early quasars.

He said, “We’ve long predicted that early quasars should drive powerful winds across their host galaxies. It’s exciting to now see clear evidence that these outflows were widespread, and even more extreme, in the early universe. The challenge now is to understand why, and what role they play in shaping these massive galaxies and their environments.”

Massive galaxies that stopped forming stars

Led by Weizhe Liu and Xiaohui Fan at the University of Arizona’s Steward Observatory and published in the journal Nature, the study could hold the key to another cosmological mystery: At very high “redshifts”—within around two billion years after the Big Bang—astronomers have turned up an unexpectedly large number of young galaxies that stopped forming stars early on.

Many of those galaxies look “old” in the sense that they had stopped forming stars long before it would be expected,” said the paper’s lead author, Weizhe Liu, a JASPER postdoctoral scholar at Steward Observatory. “How could they have formed so early and become so massive, when they quit star formation so early? That surprising discovery challenged our current paradigm of galaxy evolution, and that was one of the main motivations behind our paper.”

Extreme winds in the early universe

The likely culprits behind this process, known as “quenching,” may be the powerful winds generated by quasars now seen by James Webb. Cosmological simulations, such as those performed by Tiago Costa at Newcastle, suggest that, with their blowtorch action, quasars rid their galaxy of its gas supply, preventing it from forming stars in the host galaxy.

The team scoured the high-redshift universe for quasars and observed 27 such objects from the time of one billion years after the Big Bang. Six of them stood out through exceptionally fast galaxy-scale winds, up to 5,000 miles per second (8,400 km per second), which Liu said is extremely fast even for a quasar. The survey suggests that quasars with extremely fast outflows were at least four times more common at higher redshifts (that is, closer to the Big Bang) than at lower redshifts, and their average kinetic energy outflow rate was about 100 times higher compared to lower-redshift quasars.

“In other words, quasars with extreme outflows were much more common in the early universe and became scarcer over time, which is surprising,” said Xiaohui Fan, a Regents Professor and Associate Head of the Department of Astronomy who is the paper’s second author.

The team believes that such “super quasars” could help explain the abundance of galaxies that stopped forming stars before their time in the early universe. Although a few quasars feature a prominent particle jet, typically shooting out in opposite directions, scientists have long known that it is not the jet that blows the gas out from a galaxy, Fan explained.

“Those jets move at speeds close to the speed of light,” he said. “They essentially just punch a narrow hole into the galaxy. In contrast, the outflows we are talking about here are more like stellar winds, and we think they could be driven in many directions by radiation pressure from the quasar’s extreme bright light.”

The team also estimated that the extreme outflow quasars appear very short-lived, going dormant within about 100 million years—a cosmic blink of an eye—and leaving behind a quiescent galaxy. They estimate that every year, a galaxy with an extreme outflow quasar at its center would lose gas equivalent to thousands of solar masses.

The findings suggest that powerful quasar-driven winds may therefore have played a central role in shutting down star formation in some of the universe’s earliest massive galaxies, helping to shape the evolution of galaxies across cosmic history.