'Death by a thousand cuts': James Webb Space Telescope figures out how black hole murdered Pablo's Galaxy

Astronomers have discovered that a young galaxy was gradually starved by its central supermassive black hole, in what was effectively a cosmic "death by a thousand cuts."

The James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA) studied this unfortunate galaxy, known as GS-10578 or by the slightly snappier nickname "Pablo's Galaxy" in honor of the first astronomer to study it in detail. The light from Pablo's Galaxy has taken around 11 billion years to reach us, meaning the JWST and ALMA allow astronomers to see it as it was just 3 billion years after the Big Bang. For such an early galaxy, it is exceptionally massive, containing as much mass as around 200 billion suns.

The majority of the stars in Pablo's Galaxy seem to have formed between 12.5 billion and 11.5 billion years ago. However, this galaxy seems to have stopped forming stars and has exhausted its supply of star-forming cold gas despite its relatively young age. As astronomers define the cessation of star formation and the transition into quiescence as the "death" of a galaxy, that means Pablo's Galaxy "lived fast and died young."

The team behind this study first released results concerning Pablo's Galaxy back in Sept. 2024, using the JWST alone, finding that the supermassive black hole at its heart is pushing away huge amounts of gas at speeds as great as 2.2 million miles per hour (3.5 million km/h). That's fast enough to allow this star-forming matter to escape the gravitational influence of Pablo's Galaxy entirely.

Adding ALMA, an array of 66 radio telescopes located in the Atacama Desert region of northern Chile, the researchers observed Pablo's Galaxy for a further seven hours searching for carbon monoxide, which they could use as a way to trace cold hydrogen gas, the stuff that forms stars. However, this search turned up empty-handed.

But this in itself was telling.

"What surprised us was how much you can learn by not seeing something," team member Jan Scholtz from Cambridge University in the UK said in a statement. "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, a further 6.5 hours of observations with the JWST revealed that Pablo's Galaxy is losing about 60 suns' worth of mass in gas each year. At that rate, the galaxy's fuel for star formation could have been exhausted in a timescale of between 16 million and 220 million years. If that seems like an incredibly long period of time, consider that scientists normally estimate that it takes as long as a billion years to exhaust their fuel for star-formation in a galaxy such as this.

"The galaxy looks like a calm, rotating disc," team co-leader Francesco D'Eugenio of the Kavli Institute for Cosmology said. "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.

The team reconstructed the star formation history of Pablo's Galaxy, finding that fresh gas has been prevented by the black hole pushing gas outward from falling back into the galaxy. This prevents allowing the "fuel tanks" for star birth from being refilled. They also discovered that the supermassive black hole in this young galaxy didn't push away all of its gas at once, but has been experiencing repeated cycles of gas expulsion.

"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," D'Eugenio added.

The team's findings could help to explain why the JWST has been discovering lots of old-looking galaxies in the early universe.

"You don't need a single cataclysm to stop a galaxy forming stars, just keep the fresh fuel from coming in. Before Webb, these were unheard of," Scholtz said. "Now we know they're more common than we thought — and this starvation effect may be why they live fast and die young."

With the effectiveness of the ALMA/JWST telescope tag-team established, astronomers hope that further observations of Pablo's Galaxy can reveal more about the mechanism used by the supermassive black hole to prematurely starve this galaxy to death.

The team's research was published on Tuesday (Nov. 25) in the journal Nature Astronomy.