The discovery of bound star clusters 460 Myr after the Big Bang

Young galaxies, potentially responsible for the last major phase-transition of the Universe, appear brighter than expected and go through rapid bursty phases where copious amounts of ionizing radiation and feedback are produced. However, the stellar components of the majority of these reionization–era galaxies remain spatially unresolved. In this letter, we report the direct discovery of young massive star clusters in the strongly lensed galaxy SPT0615-JD1 (dubbed the Cosmic Gems arc) at redshift z∼10.2_-0.2^+0.2 when the universe was ∼ 460 Myr old. Recently observed with JWST/NIRCam imaging, the Cosmic Gems arc stretches over 5 (Bradley in prep.) revealing 5 individual massive young star clusters with lensing-corrected sizes of ∼1 pc, located in a region smaller than 70 pc. These Cosmic Gems produce ∼60 attenuation (A_V<0.5 mag) and metallicity (∼ 5 of ∼10^6 M_⊙, and ages younger than 35 Myr. Their stellar surface densities are around 10^5 M_⊙/pc^2, three orders of magnitude higher than typical star clusters in the local universe. Despite the uncertainties inherent to the lensing model, their dynamical ages are consistent with being gravitationally bound stellar systems that could potentially evolve into globular clusters. They would be the earliest known proto-globular clusters, formed less than 500 Myr after the Big Bang. This discovery opens a new window into the physical processes that take place in reionization-era bursty galaxies, showing that star cluster formation and clustered stellar feedback might play an important role for reionization.