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

Abstract

The Cosmic Gems arc is among the brightest and highly magnified galaxies observed at redshift z ≈ 10.2 (ref. ¹). However, it is an intrinsically ultraviolet faint galaxy, in the range of those now thought to drive the reionization of the Universe^2-4. Hitherto the smallest features resolved in a galaxy at a comparable redshift are between a few hundreds and a few tens of parsecs (pc)^5,6. 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 pc. They exhibit minimal dust attenuation and low metallicity, ages younger than 50 Myr and intrinsic masses of about 10⁶M_⊙. Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near 10⁵M_⊙ pc^-2, 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, that is, proto-globular clusters. We conclude that star cluster formation and feedback likely contributed to shaping the properties of galaxies during the epoch of reionization.