The James Webb Space Telescope discovered the first known galaxies that suddenly stopped forming stars.
A galaxy called GS-9209 stopped forming stars more than 12.5 billion years ago, researchers report Jan. 26 on arXiv.org. That’s just over a billion years after the Big Bang. Its existence reveals new details about how galaxies live and die in cosmic time.
“This is a remarkable discovery,” says astronomer Mauro Javalisco of the University of Massachusetts Amherst, who was not involved in the new study. “We really want to know when the conditions are ripe to make quenching widespread in the universe.” This study shows that at least some galaxies died out when the universe was young.
GS-9209 was first spotted in the early 2000s. In the last few years, observations with ground-based telescopes have identified it as a possible extinct galaxy based on the wavelengths of the light it emits. But Earth’s atmosphere absorbs infrared waves that can confirm the distance to a galaxy and that its star-forming days are behind it, so it was impossible to know for sure.
So astrophysicist Adam Carnall and his colleagues turned to the James Webb Space Telescope, or JWST. The observatory is very sensitive to infrared light, and it is located above the blockade of the Earth’s atmosphere. “That’s why JWST exists,” says Carnall of the University of Edinburgh. JWST also has a much higher sensitivity than earlier telescopes, allowing it to see fainter, more distant galaxies. While the largest telescopes on Earth could see GS-9209 in detail after a month of observing, “JWST can pick it up in hours.”
Using JWST observations, Carnall and his colleagues found that GS-9209 formed most of its stars over a period of 200 million years, beginning about 600 million years after the Big Bang. In that cosmically brief moment, it created about 40 billion solar masses worth of stars, roughly the size of the Milky Way.
This rapid construction suggests that GS-9209 formed from a huge cloud of gas and dust that collapsed and ignited the stars at the same time, Carnall says. “It’s pretty clear that the vast majority of stars that are out there now formed in this big burst.”
Astronomers previously believed that this method of galaxy formation, called monolithic collapse, is how most galaxies form. But this idea lost popularity, it was replaced by the idea that large galaxies are formed as a result of the slow merger of many smaller ones.
“It now appears that, at least for this object, a monolithic collapse has occurred,” Carnall says. “This is probably the strongest evidence yet that such galaxy evolution is taking place.”
As for what caused the galaxy’s frenzied star formation to suddenly stop, an active feeding black hole appears to be to blame. JWST observations revealed additional infrared light emission associated with the rapidly rotating mass of hydrogen, a sign of black hole accretion. The mass of the black hole appears to be a billion times greater than the mass of the Sun.
To reach this mass less than a billion years after the birth of the universe, a black hole would have had to feed even faster early in its life, Carnall says ( SN: 16.03.18 ). When he swallowed, he would gather a glowing disc of red-hot gas and dust around him.
“If all that radiation is coming out of the black hole, any gas that’s nearby will heat up to an incredible degree that will stop it from falling into the stars,” Carnall says.
More observations with future telescopes, such as the planned Extremely Large Telescope in Chile, could help reveal more details about how the galaxy was destroyed.
