JWST spots the earliest sign yet of a distant galaxy reshaping its cosmic environs

The James Webb Space Telescope has caught a distant galaxy blowing an unexpected bubble in the gas around it, just 330 million years after the Big Bang.

The galaxy, dubbed JADES-GS-z13-1, marks the earliest sign yet spotted of the era of cosmic reionization, a transformative period in the universe’s history when the first stars and galaxies began to reshape their environment, astronomers report in the March 27 Nature.

“It definitely puts a pin in the map of the first point where [reionization] very likely has already started,” says astrophysicist Joris Witstok at the University of Copenhagen. “No one had predicted that it would be this early” in the universe’s history.

For millions of years before JADES-GS-z13-1 and others like it began to shine, the universe was filled with cold, neutral gas, mostly hydrogen and helium. This gas absorbed short-wavelength light from any stars that shone before about 200 million years after the Big Bang. But as more and more stars began to burn and gather into galaxies, they produced enough ultraviolet light to knock electrons off the neutral gas atoms, ionizing them and making the gas transparent to short-wavelength light.

One clear signal of this ionization comes in a particular UV wavelength of light called Lyman-α, which is produced by excited hydrogen atoms returning to their lowest energy states. Seeing Lyman-α photons emanating from a galaxy means the galaxy must have blown a bubble of ionized gas around it big enough to let the particles of light reach our telescopes today.

“You can think of galaxies as little Lyman-α flashlights,” says astrophysicist Steven Finkelstein of the University of Texas at Austin, who was not involved in the new study. “If you can see the Lyman-α, it means they’re sitting in an ionized part of the universe.” If you can’t see Lyman-α, the galaxies are shrouded in neutral hydrogen fog.

Previous observations showed that the universe was completely ionized about one billion years after the Big Bang. But it’s hard to tell when the process began, or what exactly produced the light.

Witstok and colleagues used JWST to observe JADES-GS-z13-1, one of the clearest of these early galaxies, for almost 19 hours, splitting its light into a spectrum of wavelengths to seek details of the galaxy’s makeup.

JWST was designed to seek out these brilliant, ancient galaxies. As the universe expands, the ultraviolet light that these galaxies originally emitted gets stretched to longer, infrared wavelengths. Since starting operations in 2022, JWST’s sensitive infrared detectors have turned up a growing gaggle of galaxies whose light comes from as early as less than 300 million years after the Big Bang.

To their surprise, the researchers found a clear, bright signal of Lyman-α photons coming from JADES-GS-z13-1. If you were standing next to the galaxy, this light alone would shine as bright as 10 billion suns.

“We suddenly saw this huge, booming emission line” that makes all the other distant galaxies JWST has found “look a bit boring,” Witstock says. “Just the pure strength of it tells us whatever this source is has to be really, really powerful and unlike anything we’ve seen before.”

The finding is “both surprising and exciting,” says cosmologist Michele Trenti of the University of Melbourne, who was not involved in the study and wrote a perspective article that accompanied the paper in Nature. “I would not have expected the ultraviolet light that is emitted from this galaxy as Lyman-α to be able to reach the JWST,” she says. “This suggests that early forming galaxies are more efficient than previously thought at reheating the universe.”

It’s still not clear exactly what the light’s source is. The light could come from matter that was heated as it fell onto a supermassive black hole at the galaxy’s center. The galaxy’s compact size supports this idea — it looks like it’s only about 230 light-years across, compared with 32,000 light-years for the Milky Way.

The light could also have come from extremely hot, massive stars, about 100 to 300 times the mass of the sun and more than 15 times hotter. More observations are required to figure out which it is, but either one has implications for the conditions in the early universe.

“Both possibilities are stimulating for innovation,” Trenti says. “I expect theorists will be on the drawing board, developing new models for galaxy and black hole evolution during the dawn of the universe, while observers will certainly try to discover additional similar galaxies to solve the puzzle.”