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Bound star clusters observed in a lensed galaxy 460 Myr after the Big Bang

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The Cosmic Gems arc is among the brightest and highly magnified galaxies observed at redshift z 10.21. However, it is an intrinsically UV faint galaxy, in the range of those now thought to drive the reionization of the universe2–4. Hitherto the smallest features resolved in a galaxy at a comparable redshift are between a few hundreds and a few tens of parsecs5,6. Here we report JWST observations of the Cosmic Gems. The light of the galaxy is resolved into five star clusters located in a region smaller than 70 parsec. They exhibit minimal dust attenuation and low metallicity, ages younger than 50 Myr and intrinsic masses of 106 M. Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near 105 M /pc2, three orders of magnitude higher than typical young star clusters in the local universe7. Despite the uncertainties inherent to the lensing model, they are consistent with being gravitationally bound stellar systems, i.e., proto-globular clusters (proto-GCs). We conclude that star cluster formation and feedback likely contributed to 3 shape the properties of galaxies during the epoch of reionization.

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Correspondence to Angela Adamo.

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Adamo, A., Bradley, L.D., Vanzella, E. et al. Bound star clusters observed in a lensed galaxy 460 Myr after the Big Bang. Nature (2024).

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