Star Wars: What Destroyed the Colossi of the Universe?

The formation of galaxies in the universe follows, in theory, a fairly simple process. It begins with small galaxies that grow larger and larger until they become the giant galaxies we see in the present-day universe, like our Milky Way. Simple enough, isn't it?

But this isn't exactly the case with a particular class of elliptical galaxies, which are enormous spherical groups of stars without a defined structure. Under the auspices of the European Union, a team of researchers is setting out to discover the origin of these galaxies and solve more mysteries of the universe.

To do so, they've traveled back in time and used powerful telescopes capable of following light to remote corners of the universe. This has allowed scientists to observe galaxies as they were in the past, even billions of years ago.

“Galaxies are the flagpole of the universe. They are the origin of everything,” said Sune Toft, a cosmologist at the Niels Bohr Institute in Denmark. “Knowing in detail the contexts in which they formed is the only way to understand the beginnings of the universe and where we came from.”

Toft led the European ConTExt project from 2015 to 2021. His goal was to observe some of the oldest elliptical galaxies, going back to the first 2 billion years of the 13.8 billion-year history of the universe.

Time travel to remote corners: still no answers

Although researchers now have more information about elliptical galaxies, they remain a mystery. “Even though they've been known for many years, how they form remains a mystery, given that, in the local universe, they are all very old and dead,” Toft said.

The assumption behind their research is that if we look further and further back in time, observing galaxies billions of light-years away, we will eventually be able to begin to glimpse the progenitors of these galaxies and explain how they were able to grow so enormous.

“But no matter how far back we went in our observations, they still looked like old, dead galaxies. They exhibit virtually no star formation,” Toft said, referring to the central process of galaxy evolution. This means that galaxies must have grown very rapidly in the early Universe. Even so, we still don't know exactly how and when.

And this raises another puzzle: If galaxies grew rapidly, why did they stop? And what did that mean for our understanding of the hierarchical structure of galaxies in the universe, which consists of stars, planetary systems, star clusters, and galaxies? “Small galaxies are supposed to form first; so how come these massive galaxies were the first to form?” Toft asked.

Star formation

Their hypothesis is that these galaxies may have experienced intense star formation early in their history, becoming what are known as starburst galaxies. Starburst galaxies contain extremely dense amounts of dust and gas and can form stars thousands of times more massive than our Sun each year. By comparison, our Milky Way forms one new solar mass per year, on average.

Toft worked with a telescope in Chile called the Atacama Large Millimeter Array , as well as the Hubble and Spitzer space telescopes , which were orbiting Earth at the time. He found that between the first and second billion years after the Big Bang, “there were enough star-forming galaxies to become the dead galaxies.” These galaxies were dense and compact and resembled the cores of the elliptical galaxies we see today.

Toft's work was based on the premise that these progenitor elliptical galaxies formed rapidly in the universe before something halted their star formation. Then, over the next 10 billion years or so, these galaxies gradually accreted more stars, swallowing smaller galaxies, whose stars were added to their outlying regions. Thus, the elliptical galaxies were still old and dead, but they could still grow to immense size.

The early growth of ellipticals was likely caused by galaxy mergers that triggered star formation. “Two large galaxies collide, and the gas is compressed at the center of the collision,” Toft explained. “That’s what it takes to have very high star formation rates.”

But what was still unclear was how these galaxies died out. Why did they stop forming stars so quickly and end up becoming the dead galaxies we see today?

The shutdown

Sirio Belli, an astronomer at the University of Bologna in Italy, is investigating this problem as part of his Cardinal Network project, an EU-funded initiative that began in 2023 and will continue until 2028.

Using the powerful James Webb Space Telescope ( JWST ) orbiting the Sun, we are tracking these early galaxies like never before. The emerging idea is that the black holes at the centers of these galaxies are responsible for their evolution. Today, nearly every galaxy, including our own, contains a supermassive black hole at its center—an enormous object that may be millions to billions of times the mass of our Sun. These black holes drive galaxy formation and evolution, and they combust and eject gas and dust throughout a galaxy’s history.

Belli has discovered that these black holes could also be responsible for the interruption of star formation in primitive galaxies, due to a process known as quenching. ( quenching ).

In April 2024, his team used JWST to report the discovery of a massive galaxy that was in the process of fading away roughly 2.6 billion years after the Big Bang. “It was a lucky coincidence to be able to observe this galaxy right as it was fading away,” he commented.

The galaxy appeared to have been growing until recently. “It simply stopped forming stars,” Belli said. “At the same time, we discovered powerful winds emanating from the galaxy. We think this is due to the supermassive black hole at its center.”

The idea, according to Belli, is that the black hole became extremely active, which "expelled gas from the galaxy." "So there's no more gas to form new stars. It's like a car that runs out of gas."

What remains unclear is the exact reason why the black hole became active. One possibility is that, when the black hole engulfs enough material and acquires enough mass, it suddenly begins to emit a lot of energy, causing it to shut down.

“We believe that once galaxies reach a certain mass, 100 billion solar masses, they all eventually go dark,” Belli reported. “We don’t observe any massive galaxies in the current universe that are still forming stars.”

An extremely large telescope for further research

More answers could come from new telescopes like the European Extremely Large Telescope (ELT) , which is being built in Chile and is scheduled to begin observations in 2028.

“With the ELT, we can observe the interiors of these galaxies in detail” in the early universe, Belli noted, something the JWST cannot offer.

This would tell researchers the overall rate of star formation, but also "where stars are forming," he commented. "If the ELT works as promised, it will be amazing." Determining how the quenching process works is crucial to solving the mystery of why galaxies die, a question that continues to perplex scientists.

“It shouldn't be possible, because when a galaxy is in the early universe, it's filled with gas,” Toft noted. “How do you go from forming thousands of solar masses a year to nothing? If we want to prove that black holes are responsible, we have to find galaxies in the process of shutting down.” If we can understand this, we'll know how the cosmos came to be as we know it today.

The research described in this article was funded by the European Research Council (ERC). The views of the interviewees do not necessarily reflect those of the European Commission. If you enjoyed this article, please share it on your social media.

This article originally appeared in Horizon , the European Union's research and innovation magazine.