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

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 10 ⁶ M _⊙ . Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near 10 ⁵ M _⊙  pc ^-2 , three orders of magnitude higher than typical young star clusters in the local Universe ⁷ . 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.
(© 2024. The Author(s).)