An early hint of cosmic dawn has been seen in a distant galaxy

A galaxy found at the dawn of the universe appears to be the earliest known evidence of cosmic reionisation, the period when the universe was lit up for the first time.

Following the big bang, the early universe was filled with hot hydrogen and helium gas that scattered photons, making the cosmos somewhat opaque. Over the next few hundred million years, as stars began to shine, their light ionised the hydrogen and helium, enabling photons to flow freely and making the universe transparent, though the exact timing of this is uncertain.

Joris Witstok at the University of Copenhagen in Denmark and his colleagues used the James Webb Space Telescope (JWST) to study a galaxy called JADES-GS-z13-1-LA. The galaxy is seen 330 million years after the big bang, making it one of the earliest known galaxies in the universe.

Ultraviolet light from the galaxy suggests it was surrounded by a bubble about 200,000 light years across, which might be the result of its starlight interacting with the surrounding cosmic hydrogen. Seeing evidence for this so early in the universe is “beyond even our wildest expectations,” says Witstok.

Michele Trenti at the University of Melbourne agrees that the observations are consistent with the process of cosmic reionisation. “It’s both surprising and exciting,” says Trenti. “I would not expect the ultraviolet light emitted from this galaxy to reach JWST. The cold neutral hydrogen gas that we were expecting would have surrounded the galaxy should have blocked the photons. We are witnessing the onset of reionisation.”

The nature of the small galaxy itself is not entirely clear; it might be shining brightly because of a population of massive hot and young stars, or a powerful central black hole. “This would be the earliest known evidence for a supermassive black hole at the centre of a galaxy,” says Trenti.

While astronomers have seen other, later galaxies with a similar bubble around them, JADES-GS-z13-1-LA is the earliest known example. “It’s a benchmark,” says Richard Ellis at University College London. “It tells us that this galaxy must have been around for quite a while, and pushes that little bit further back to the beginning of when galaxies first emerged from darkness.”

JWST was able to unearth the secrets of this galaxy only by staring at it for a relatively long time, about 19 hours. Witstok is hopeful we might soon see other early evidence for cosmic reionisation. “We have a few more candidates,” he says. “We might find it even further [back in time], or maybe this is the most extreme that it gets.”