Astronomers May Have Just Spotted the Universe’s First Galaxies

Scientists just announced that they’ve detected what might be some of the earliest galaxies to form in the universe, a tantalizing discovery made thanks to NASA’s new flagship James Webb Space Telescope. 

“This is the first large sample of candidate galaxies beyond the reach of the Hubble Space Telescope,” its predecessor, said Haojing Yan, a University of Missouri astronomer who led the newly published study, at a press conference yesterday at the American Astronomical Society meeting in Seattle. Because the more sensitive JWST can see further into deep space than Hubble does, it essentially sees further back in time. In the new catalog of 87 galaxies astronomers have spotted with it, some could date back to about 13.6 billion years ago, just 200 million years after the Big Bang. That’s when the galaxies emitted the light that we’re seeing today—although those systems of stars, gas, and dust would have changed dramatically since then, if they still exist at all.

While scientists have studied other faraway galaxies that date to when the universe was still young, Yan’s and his colleagues’ discoveries could break those records by a few hundred million years or so. But at this point, they’re all still considered “candidate galaxies,” which means that their birthdates still need confirmation. 

Dating a galaxy can be a challenging matter: It involves measuring its “redshift,” how much the light it emits is stretched towards longer red wavelengths, which tells astronomers how fast the galaxy is moving away from us in the quickly expanding universe. That, in turn, tells astronomers its distance from Earth—or more exactly, the distance that the photons from its stars had to travel at the speed of light before reaching a space telescope near the Earth, like JWST. Light from stars in the most distant galaxy in this collection may have been emitted 13.6 billion years ago, likely fairly soon after the young galaxy came together. 

These newly-estimated galaxies’ distances will have to be confirmed with spectra, which means measuring the light they emit across the electromagnetic spectrum and pinpointing its unique signatures. Still, Yan expects many of them to be correctly dated to the early days of the cosmos: “I’ll bet $20 and a tall beer that the success rate will be higher than 50 percent,” he said.

Yan’s team imaged them with JWST’s NIRCam at six near-infrared wavelengths. To estimate their distances, the astronomers used a standard “dropout” technique: Hydrogen gas surrounding galaxies absorbs light at a particular wavelength, so the wavelengths at which an object can or can’t be seen puts a limit on how far away it is likely to be. These 87 candidate galaxies mostly look like blobs that can only be detected in the longer (and therefore redder) near-infrared wavelengths detectable by NIRCam, which could mean they’re very distant, and therefore very old. 

However, it’s possible that some of them could be much closer than expected—which would mean they aren’t so old after all. For example, it could be that their light is just too faint to be detected at some wavelengths. Until Yan can collect more detailed data, he won’t know for sure.