Webb Finds Proto-Globular Clusters in Early Universe

Astronomers using the NASA/ESA/CSA James Webb Space Telescope have spotted at least five young globular clusters within SPT 0615-JD1 (also known as the Cosmic Gems arc), a strongly-lensed galaxy that existed when the Universe was 460 million years old.

“These galaxies are thought to be a prime source of the intense radiation that reionized the early Universe,” said Dr. Angela Adamo, an astronomer at Stockholm University and the Oskar Klein Centre.

“What is special about the Cosmic Gems arc is that thanks to gravitational lensing we can actually resolve the galaxy down to parsec scales.”

SPT 0615-JD1 was initially discovered in Hubble images obtained by the RELICS (Reionization Lensing Cluster Survey) program of the lensing galaxy cluster SPT-CL J0615-5746, which is located approximately 7.7 billion light-years away in the constellation of Pictor.

With Webb, Dr. Adamo and colleagues can now see where stars formed and how they are distributed, in a similar way to how Hubble is used to study local galaxies.

Webb’s view provides a unique opportunity to study star formation and the inner workings of infant galaxies at such an unprecedented distance.

“Webb’s incredible sensitivity and angular resolution at near-infrared wavelengths, combined with gravitational lensing provided by the massive foreground galaxy cluster, enabled this discovery. No other telescope could have made this discovery,” said Dr. Larry Bradley, an astronomer at the Space Telescope Science Institute

“The surprise and astonishment was incredible when we opened the Webb images for the first time,” Dr. Adamo said.

“We saw a little chain of bright dots, mirrored from one side to the other — these cosmic gems are star clusters.”

“Without Webb we would not have known we were looking at star clusters in such a young galaxy.”

The astronomers note that their discovery connects a variety of scientific fields.

“These results provide direct evidence that indicates proto-globular clusters formed in faint galaxies during the reionization era, which contributes to our understanding of how these galaxies have succeeded in reionizing the Universe,” Dr. Adamo said.

“This discovery also places important constraints on the formation of globular clusters and their initial properties.”

“For instance, the high stellar densities found in the clusters provide us with the first indication of the processes taking place in their interiors, giving new insights into the possible formation of very massive stars and black hole seeds, which are both important for galaxy evolution.”

In the future, the team hopes to build a sample of galaxies for which similar resolutions can be achieved.

“I am confident there are other systems like this waiting to be uncovered in the early Universe, enabling us to further our understanding of early galaxies,” said Dr. Eros Vanzella, an astronomer at the INAF – Astrophysics and Space Science Observatory of Bologna.

The findings appear today in the journal Nature.

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A. Adamo et al. Bound star clusters observed in a lensed galaxy 460 Myr after the Big Bang. Nature, published online June 24, 2024; doi: 10.1038/s41586-024-07703-7