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

TL;DR

This study uses JWST observations to resolve star clusters in a galaxy from 460 million years after the Big Bang, revealing proto-globular clusters with high stellar densities that likely influenced early galaxy evolution.

Contribution

First direct observation of star clusters at such high redshift, showing they are compact, massive, and potentially gravitationally bound, informing early galaxy formation models.

Findings

Resolved five star clusters in a z~10.2 galaxy with sizes ~1 pc

Clusters have high stellar surface densities (~10^5 M_sun/pc^2)

Evidence suggests these are proto-globular clusters

Abstract

The Cosmic Gems arc is among the brightest and highly magnified galaxies observed at redshift $z\sim10.2$. However, it is an intrinsically UV faint galaxy, in the range of those now thought to drive the reionization of the Universe. Hitherto the smallest features resolved in a galaxy at a comparable redshift are between a few hundreds and a few tens of parsecs. 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 $\sim10^6$ M$_{\odot}$. Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near $10^5$~M$_{\odot}$/pc$^2$, three orders of magnitude higher than typical young star clusters in the local universe. Despite the uncertainties inherent to the lensing model, they are consistent with being gravitationally bound stellar systems, i.e., proto-globular clusters. We conclude that star cluster formation and feedback likely contributed to shape the properties of galaxies during the epoch of reionization. [Abridged]