JD1: a galaxy from the primordial universe

An international research team led by UCLA has discovered the galaxy JD1, one of the oldest in the primordial universe
Image captured by the James Webb Space Telescope. In the inset, the magnification of the galaxy JD1, one of the earliest detected in the primordial universe. Credit: NASA

After the Big Bang, the universe expanded and cooled sufficiently for hydrogen atoms to form. In the absence of light from the first stars, the universe entered a period known as the cosmic dark ages. The first stars and galaxies appeared several hundred million years later. They began burning the leftover hydrogen from the Big Bang, making the universe “transparent” as it is today.

An international research group led by astrophysicists from UCLA has confirmed the existence of JD1, the faintest galaxy ever seen in the primordial universe. The galaxy is one of the most distant identified to date and is precisely the type that burned off the fog of hydrogen atoms left by the Big Bang. It was in this process that light began to shine through the universe and shape it into what it is today. The discovery was made using NASA‘s James Webb Space Telescope, and the results were published in the journal Nature (ref).

Reionization Process

The first billion years of the universe’s life were a crucial period in its evolution. After the Big Bang approximately 13.8 billion years ago, the universe expanded and cooled enough to allow the formation of hydrogen atoms. However, until the birth of the first stars and galaxies, the universe was dark. The appearance of the first stars and galaxies a few hundred million years later flooded the cosmos with energetic ultraviolet light that began to burn off the hydrogen fog.

Determining the types of galaxies that dominated that era, known as the Epoch of Reionization, is one of the main goals of modern astronomy. But until the development of the James Webb Telescope, scientists lacked the sensitive infrared tools needed to study the first generation of galaxies.

“Most of the galaxies found so far by the James Webb Telescope are bright and rare. But not considered particularly representative of the young galaxies that populated the primordial universe” said Guido Roberts-Borsani, the study’s lead author. “Ultra-faint galaxies like JD1, on the other hand, are much more numerous. For this reason, we believe they are more representative of the galaxies that drove the reionization process, allowing ultraviolet light to travel unobstructed through space and time.”

James Webb and the Gravitational Lens

JD1 is such a faint and distant galaxy in the primordial universe that it would be challenging to study without a powerful telescope and a natural trick. JD1 is located behind a massive cluster of nearby galaxies, Abell 2744. The cluster acts as a gravitational lens, amplifying the light from JD1, making it 13 times brighter than it would otherwise be. Without this effect, the discovery of JD1 would likely have been missed.

For the analysis, the researchers used the NIRSpec spectrograph to obtain an infrared light spectrum of the galaxy. This allowed them to determine its precise age and distance from Earth, as well as the number of stars and the amount of dust and heavy particles.

The combination of gravitational magnification and the new images captured by James Webb’s NIRCam allowed the team to study the galaxy’s structure with unprecedented detail and resolution. Since light takes time to reach us, JD1 appears as it was approximately 13.3 billion years ago when the universe was only about 4% of its current age.

“Before the Webb telescope turned on we couldn’t even dream of confirming such a faint galaxy” said Tommaso Treu, the study’s second author. “The combination of JWST and the power of gravitational lenses is a revolution. We are rewriting the book on how galaxies formed and evolved shortly after the Big Bang”.

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