The Pablo 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 D’Eugenio wasn’t looking for a murdered galaxy.
His team set out to measure the movements of stars in a distant galaxy and understand why they appeared so old. Similar massive galaxies in the early universe were typically filled with new star formations, and D’Eugenio, a scientist at the Kavli Institute for Cosmology in Cambridge, wondered whether dust could obscure young stars from view.
Instead, they found that young stars were missing entirely. The galaxy had shut down.
Their results, published in Natural astronomy on September 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), D’Eugenio and an international team of astronomers observed the Pablo Galaxy, a massive and distant system the size of the Milky Way, extremely compact and measuring approximately 200 billion times mass of our Sun.
Astronomers can peer into the early universe by studying light from distant galaxies. As the universe expands, the light from these galaxies stretches, shifting toward the red end of the spectrum in a phenomenon known as redshift. The Pablo Galaxy has a redshift of about 3, which means that on Earth we see it as it was about 11.7 billion years ago, when the universe was only 2 billion years old and star formation was at its peak. its peak.
“It is safe to say that this light is older than the Sun and the Earth,” says D’Eugenio.
At the center of the Pablo Galaxy is the supermassive black hole. When black holes like this become active, they can release large amounts of radiation and powerful winds that reshape their galaxy. In the Pablo Galaxy, researchers have found that its black hole ejects cold, dark gas at speeds of up to 1,000 kilometers per second, pushing it out of the galaxy’s gravitational reach. Without this gas, the galaxy cannot form new stars, a condition scientists call “quenched.”
“We knew there was a black hole, but we didn’t expect it to have such a massive effect on a galaxy of this size and with these properties,” D’Eugenio says.
With its advanced sensitivity and resolution in the near- and mid-infrared wavelengths, JWST allowed researchers to observe this quenching process in unprecedented detail. The telescope revealed a new component of the black hole’s wind: dense, cold gas that emits no light, making it invisible to previous instruments. JWST was able to detect these dark gas clouds by observing how they block some of the light from the galaxy behind them, offering new insights into how black holes eject star-forming material – a key to understanding the life cycles of galaxies in the cosmos.
A temporary destiny?
As it starves its supermassive black hole, the Pablo Galaxy may be dozing rather than dead. “It’s not a set fate when galaxies die out,” says Olivia Cooper, a doctoral student in astrophysics. student at the University of Texas at Austin, who was not involved in the work. Under the right conditions, the galaxy could reacquire gas and potentially reignite star formation in the future.
Despite the dramatic halt in star formation, the study found that Pablo’s Galaxy retains an unexpected feature: a rotating, well-organized stellar disk, just like our Milky Way. This calls into question previous hypotheses about how galaxies evolve after extinction. In the local universe – the region of space relatively close to us on a cosmic scale – extinct galaxies typically lose their ordered structure and become more chaotic. But the Pablo Galaxy has maintained a stable, disk-like rotation, with stars orbiting cohesively around the galaxy’s center, preserving the flat, structured motion characteristic of spiral galaxies.
“This is really amazing,” says Minjung Park, a Ph.D. student at the Harvard and Smithsonian Center for Astrophysicals in Cambridge, Massachusetts, who was also not involved in the work. “It means that the extinction mechanisms do not destroy the stellar disk, but somehow interrupt star formation.”
Because extinct galaxies like this one are not seen in the local universe, researchers suspect that something has changed in the 11 billion years since they observed the Pablo Galaxy. Understanding what happens to these galaxies over time could reveal new insights into the early universe and the evolution of galaxies.
Looking ahead, D’Eugenio and his team plan to expand their search by looking for galaxies that have only recently become extinct, where they can observe more recent signs of gas ejection, and study lower-mass galaxies to better understand when and why galaxies stop. form stars.
“At the moment, we see this happening mostly in more massive galaxies. We think it’s because they have the most massive black holes, but maybe it’s just because we’ve observed the brightest objects so far,” explains D’Eugenio. “We want to push down and understand where this process begins.”
