2023MNRAS.520.2180C

We present the clump populations detected in 18 lensed galaxies at redshifts 1–8.5 within the lensing cluster field SMACS0723. The recent JWST Early Release Observations of this poorly known region of the sky have revealed numerous point-like sources within and surrounding their host galaxies, undetected in the shallower Hubble Space Telescope images. We use JWST multi-and photometry and the lensing model of this galaxy cluster to estimate the intrinsic sizes and magnitudes of the stellar clumps. We derive optical restframe effective radii from <10 to hundreds pc and masses ranging from ∼10⁵ to 10⁹ M _⊙, overlapping with massive star clusters in the local universe. Clump ages range from 1 Myr to 1 Gyr. We compare the crossing time to the age of the clumps and determine that between 45 and 60 per cent of the detected clumps are consistent with being gravitationally bound. On average, the dearth of Gyr old clumps suggests that the dissolution time scales are shorter than 1 Gyr. We see a significant increase in the luminosity (mass) surface density of the clumps with redshift. Clumps in reionization era galaxies have stellar densities higher than star clusters in the local universe. We zoom in into single galaxies at redshift <6 and find for two galaxies, the Sparkler and the Firework, that their star clusters/clumps show distinctive colour distributions and location surrounding their host galaxy that are compatible with being accredited or formed during merger events. The ages of some of the compact clusters are between 1 and 4 Gyr, e.g. globular cluster precursors formed around 9–12 Gyr ago. Our study, conducted on a small sample of galaxies, shows the potential of JWST observations for understanding the conditions under which star clusters form in rapidly evolving galaxies.