The Epoch of Reionization (EoR), within the first billion years after the big bang, saw the universe's transition from neutral hydrogen gas to ionized matter. Early galaxies are believed to have driven this change. Studying these galaxies requires observing distant objects, allowing us to "look back in time." Gravitational lensing, where a massive celestial body bends light like a magnifying glass, helps observe distant galaxies in detail.

The Cosmic Gems arc as observed by the JWST. Credit: ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration.

In a study published in Nature, the team of astronomers found star clusters in the galaxy Cosmic Gems arc through gravitational lensing. “This achievement could only be possible thanks to JWST unmatched capabilities”, says Dr. Adélaïde Claeyssens, co-author of the article. The JWST's Near Infrared Camera (NIRCam) revealed five young massive star clusters. These clusters, three times denser than typical young clusters, formed within the last 50 million years and make up 60% of the galaxy's total light.

It was incredible to see the JWST images of the Cosmic Gems arc and realise that we were looking at star clusters in such a young galaxy.

"We observe globular clusters around local galaxies, but we don’t know when and where they formed. The Cosmic Gems arc observations have opened a unique window for us into the works of infant galaxies as well as showing us where globular clusters formed”, says Adamo. These clusters are expected to evolve into globular clusters over time.

Left: a negative version of the star clusters, where the different star clusters are marked. Right: the star clusters “behind” the gravitational lens. This image was calculated using computer simulations. Credit: ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration.

The team plans to study more galaxies from the EoR to understand early star cluster demographics. They also have an approved JWST program to further examine Cosmic Gems arc and its star clusters. “Spectroscopic observations will allow us to spatially map the star formation rate and ionizing photon production efficiency along the galaxy”, says Dr. Larry Bradley, principal investigator of the JWST program and the second author of this article.

The article “Bound star clusters observed in a lensed galaxy 460 Myr after the Big Bang” is published in Nature. Dr. Angela Adamo is the lead author, with Dr. Adélaïde Claeyssens (Stockholm University) and Erik Zackrisson (Uppsala University) as co-authors.

Press release Nature