Pablo’s Galaxy is a distant galaxy in the early universe whose central supermassive black hole is starving it of the gas needed to create new stars. Credit: JADES Collaboration
Astronomer Francesco Dâ€Eugenio wasnâ€t looking for a murdered galaxy.
His team set out to measure the motions of stars in a distant galaxy and to understand why they appeared so old. Similar massive galaxies in the early universe were typically bustling with new star formation, and Dâ€Eugenio, a scientist at the Kavli Institute for Cosmology in Cambridge, wondered if dust might be obscuring the young stars from view.
Instead, they found that the young stars were missing entirely. The galaxy had shut down.
Their findings, published in Nature Astronomy on Sept. 16, offer direct evidence of a black hole starving its host galaxy of the gas needed to form new stars, supporting a long-theorized but elusive mechanism behind galaxy death.
Peering back in time
Using data from the James Webb Space Telescope (JWST), Dâ€Eugenio and an international team of astronomers observed Pabloâ€s Galaxy — a massive, distant Milky Way-sized system that is extremely compact, measuring about 200 billion times the mass of our Sun.
Astronomers can peer into the early universe by studying light from distant galaxies. As the universe expands, light from these galaxies stretches, shifting toward the red end of the spectrum in a phenomenon known as redshift. Pabloâ€s Galaxy has a redshift of about 3, meaning that on Earth, weâ€re seeing it as it was roughly 11.7 billion years ago, when the universe was just 2 billion years old and star formation was at its peak.
“Itâ€s safe to say this light is older than the Sun and Earth,†Dâ€Eugenio says.
At the center of Pabloâ€s Galaxy lies its supermassive black hole. When black holes like this become active, they can unleash vast amounts of radiation and powerful winds that reshape their galaxy. In Pabloâ€s Galaxy, researchers found its black hole ejecting cold, dark gas at speeds up to 620 miles (1,000 kilometers) per second, blowing it out of the galaxyâ€s gravitational reach. Without this gas, the galaxy canâ€t form new stars — a condition scientists called “quenched.â€
“We knew there was a black hole, but we didnâ€t expect it to have this massive effect on a galaxy of this size and with these properties,†Dâ€Eugenio says.
With its advanced sensitivity and resolution in near- and mid-infrared wavelengths, JWST allowed the researchers to observe this quenching process in unprecedented detail. The telescope revealed a new component of the black holeâ€s wind: dense, cold gas that doesnâ€t emit light, making it invisible to previous instruments. JWST could detect these dark gas clouds by observing how they blocked some of the galaxyâ€s light behind them, offering new insights into how black holes expel star-forming material — a key to understanding the life cycles of galaxies across the cosmos.
A temporary fate?
While starved by its supermassive black hole, Pabloâ€s Galaxy may be napping rather than dead. “Itâ€s not a definite fate when galaxies are quenched,†says Olivia Cooper, an astrophysics Ph.D. student at the University of Texas at Austin, who wasnâ€t involved in the work. Under the right conditions, the galaxy could reacquire gas and potentially reignite star formation in the future.
Despite its dramatic shutdown in star formation, the study found that Pabloâ€s Galaxy retains an unexpected feature: a well-organized, rotating stellar disk, much like our own Milky Way. This challenges previous assumptions about how galaxies evolve after quenching. In the local universe — the region of space relatively close to us on a cosmic scale — quenched galaxies typically lose their orderly structure and become more chaotic. But Pabloâ€s Galaxy has maintained a stable, disklike rotation, with stars orbiting cohesively around the galaxyâ€s center, preserving the flat, structured movement characteristic of spiral galaxies.
“This is really surprising,†says Minjung Park, a Ph.D. student at the Harvard and Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was also not involved in the work. “It means that the quenching mechanisms do not destroy the stellar disk, but somehow shut down star formation.â€
Because quenched galaxies like this arenâ€t seen in the local universe, the researchers suspect something must have changed over the 11 billion years since they observed Pabloâ€s Galaxy. Understanding what happens to these galaxies over time could reveal new insights into the early universe and galaxy evolution.
Looking ahead, Dâ€Eugenio and his team plan to expand their research by searching for galaxies that have only recently been quenched, where they can observe fresher signs of gas expulsion, and to study lower-mass galaxies to better understand when and why galaxies stop forming stars.
“At the moment, we see this happening mainly in the most massive galaxies. We think itâ€s because they have the most massive black holes, but perhaps itâ€s just that, so far, weâ€ve been looking at the brightest objects,†Dâ€Eugenio explains. “We want to push down and understand where this process starts.â€
