James Webb Space Telescope finds the faintest galaxy ever detected at the dawn of the universe

The James Webb Space Telescope (JWST) has identified one of the most distant galaxies ever seen — an ancient, nearly invisible star cluster so remote that its light is the faintest scientists have ever detected.

Called JD1, the galaxy — whose light traveled for roughly 13.3 billion years to reach us — was born just a few million years after the Big Bang. Back then, the cosmos was shrouded in a pitch-black fog that not even light could pass through; galaxies like this one were vital in burning the gloom away.

Twinkling from within the Sculptor constellation in the southern sky, JD1's light left its source when the universe was just 4% of its current age. The light crossed dissipating gas clouds and boundless space before passing through the galaxy cluster Abell 2744, whose space-time-warping gravitational pull acted as a giant magnifying lens to steer the ancient galaxy into focus for the JWST. The researchers who discovered the dim, distant galaxy published their findings May 17 in the journal Nature.

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"Before the Webb telescope switched on, just a year ago, we could not even dream of confirming such a faint galaxy," Tommaso Treu, a physics and astronomy professor at the University of California, Los Angeles (UCLA), said in a statement. "The combination of JWST and the magnifying power of gravitational lensing is a revolution. We are rewriting the book on how galaxies formed and evolved in the immediate aftermath of the Big Bang."

In the first hundreds of millions of years after the Big Bang, the expanding universe cooled enough to allow protons to bind with electrons, creating a vast shroud of light-blocking hydrogen gas that blanketed the cosmos in darkness. From the eddies of this cosmic sea-foam, the first stars and galaxies clotted, beaming out ultraviolet light that reionized the hydrogen fog, breaking it down into protons and electrons to render the universe transparent again.

Astronomers have observed evidence for reionization in many places: the dimming of brightly flaring quasars (ultrabright objects powered by supermassive black holes); the scattering of light from electrons in the cosmic microwave background; and the infrequent, dim light given off by hydrogen clouds. Yet because the first galaxies used so much of their light to dissipate the stifling hydrogen mist, what they actually looked like has long remained a mystery to astronomers.   

"Most of the galaxies found with JWST so far are bright galaxies that are rare and not thought to be particularly representative of the young galaxies that populated the early universe," first author Guido Roberts-Borsani, an astronomer at UCLA, said in the statement. "As such, while important, they are not thought to be the main agents that burned through all of that hydrogen fog.

"Ultra-faint galaxies such as JD1, on the other hand, are far more numerous, which is why we believe they are more representative of the galaxies that conducted the reionization process, allowing ultraviolet light to travel unimpeded through space and time," Roberts-Borsani added.

To discover JD1's first stirrings from beneath its hydrogen cocoon, the researchers used the JWST to study the galaxy's gravitationally lensed image in the infrared and near-infrared spectra of light. This enabled them to detect JD1's age, distance from Earth and elemental composition, as well as estimate how many stars it had formed. The team also made out a trace of the galaxy's structure: a compact glob built from three main spurs of star-birthing gas and dust.  

The astronomers' next task is to use their technique to unveil even more of these first galaxies, revealing how they worked in unison to bathe the universe in light.