Astronomers have spotted one of the oldest ‘dead’ galaxies yet identified, and found that a growing supermassive black hole can slowly starve a galaxy rather than tear it apart.

The researchers, led by the University of Cambridge, used data from the James Webb Space Telescope and the Atacama Large Millimeter Array (ALMA), to study a galaxy in the early universe – about three billion years after the Big Bang.

The galaxy, called GS-10578 but nicknamed ‘Pablo’s Galaxy’ after the astronomer who first observed it in detail, is massive for such an early period in the universe: about 200 billion times the mass of our Sun, and most of its stars formed between 12.5 and 11.5 billion years ago.

Pablo’s Galaxy appears to have ‘lived fast and died young’: it stopped forming new stars, despite its relatively young age, due to an almost total absence of the cold gas stars need to form.

The supermassive black hole at the galaxy’s centre appears to be the culprit. But instead of a single cataclysmic event, the galaxy suffered ‘death by a thousand cuts’ as the black hole repeatedly heated the gas in and around the galaxy, preventing it from resupplying the galaxy with fresh gas and slowly strangling star formation. The results are reported in the journal Nature Astronomy.

The researchers spent nearly seven hours observing the galaxy with ALMA, hoping to detect carbon monoxide – a tracer of cold hydrogen gas. Instead, they found nothing.

“What surprised us was how much you can learn by not seeing something,” said co-first author Dr Jan Scholtz from Cambridge’s Cavendish Laboratory and Kavli Institute for Cosmology. “Even with one of ALMA’s deepest observations of this kind of galaxy, there was essentially no cold gas left. It points to a slow starvation rather than a single dramatic death blow.”

Meanwhile, JWST spectroscopy revealed powerful winds of neutral gas streaming out of the galaxy’s supermassive black hole at 400 kilometres per second, removing 60 solar masses of gas every year. Those numbers suggest the galaxy’s remaining fuel was depleted in as little as 16 to 220 million years – far faster than the billion-year timescale typical for similar galaxies.

“The galaxy looks like a calm, rotating disc,” said co-first author Dr Francesco D’Eugenio, who is also affiliated with the Kavli Institute for Cosmology. “That tells us it didn’t suffer a major, disruptive merger with another galaxy. Yet it stopped forming stars 400 million years ago, while the black hole is yet again active. So the current black hole activity and the outburst of gas we observed didn’t cause the shutdown; instead, repeated episodes likely kept the fuel from coming back.”

By reconstructing the galaxy’s star-formation history, the researchers concluded that the galaxy evolved with net-zero inflow – meaning fresh gas never refilled its tank. Rather than blowing away all its gas in one go, the black hole seems to have heated or expelled incoming material over multiple cycles, preventing the galaxy from replenishing itself.

“You don’t need a single cataclysm to stop a galaxy forming stars, just keep the fresh fuel from coming in,” said Scholtz.

The findings help explain a growing population of massive, surprisingly old-looking galaxies seen by Webb in the early Universe. “Before Webb, these were unheard of,” said Scholtz. “Now we know they’re more common than we thought – and this starvation effect may be why they live fast and die young.”

The study shows the advantages of combining ALMA’s ultra-deep radio observations with JWST’s infrared spectra. Future work will target more galaxies like this one to see whether slow starvation, rather than violent blowouts, is the norm for galaxies in the early universe.

The Cambridge team was awarded additional 6.5 hours of JWST time using the MIRI instrument. These new observations targeting the warmer hydrogen gas will tell us more about the exact mechanisms that this supermassive black hole is using to stop the galaxy from forming stars.