The supermassive black holes at the centre of many galaxies were suspected to quench the formation of new stars – now the James Webb Space Telescope has spotted evidence of this.
Supermassive black holes seem to stop stars forming in their enormous host galaxies – and the James Webb Space Telescope (JWST) may have caught them in the act for the first time.
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| Illustration of a supermassive black hole Stuart Thomson/Alamy |
Many of the most massive galaxies in our cosmic neighbourhood are in a “quiescent” state, where no new stars seem to have been born for a long time. Astronomers have long suspected that the supermassive black holes at the centre of many galaxies might be responsible by somehow disturbing the clouds of gas that would make new stars, in a process called quenching. But catching this process in action has proved difficult as it happened so long ago.
Now, two separate teams of astronomers have observed streams of gas pouring out of galaxies with supermassive black holes, using JWST to peer back in time to more than 10 billion years ago. “I would say it’s the first direct evidence that supermassive black holes can actually remove the gas in a way that turns off the galaxy,” says Sirio Belli at the University of Bologna in Italy.
Belli and his colleagues trained JWST’s spectroscope on a galaxy called COSMOS-11142, which had very recently ceased forming new stars, and analysed the gas flowing out of it. They observed both hot, ionised gas and much colder, neutral gas, which is hard to detect from Earth-based telescopes.
“With the neutral gas, we see that actually there is a lot of mass leaving the galaxy,” says Belli. “So it’s a very natural conclusion that this wind is then turning off the galaxy, because, of course, the gas is the fuel for star formation.”
Francesco D’Eugenio at the University of Cambridge and his colleagues observed a different galaxy, GS-10578, from around 11 billion years ago. Using a different spectroscope from JWST, they measured star formation rates and concluded that the galaxy had stopped forming stars relatively recently, only a few tens of millions of years ago, relative to how we see it now.
They also measured the neutral gas flowing out of the galaxy and found that its mass was the same order of magnitude as the gas required to form new stars. “This means that these outflows can efficiently compete with star formation for using up the gas,” says D’Eugenio.
Both these observations provide strong evidence that a black hole can suppress star formation in its galaxy, says Omar Almaini at the University of Nottingham, UK, but it isn’t conclusive and there is a possibility that other mechanisms are at play. “It could be that the black hole just happens to be active at this critical stage in the evolution of the galaxy, that the black hole is just along for the ride.”
It is also possible that, even if a black hole is suppressing star formation, it is doing it through means other than removing the gas, such as by introducing turbulence to the pools of gas, says Almaini. Many more observations of a larger sample of galaxies will be needed to reach a conclusion, he says. “It’s going to require a lot of detective work pulling together bits of independent evidence to point to the same thing.”
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arXivDOI: 10.48550/arXiv.2308.05795
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