Marta Reina-Campos

JWST imaging of the rich galaxy cluster Abell 2744 at z = 0.308 has been used by the UNCOVER team to construct mosaic images in the NIRCam filters. The exceptionally deep images in the (F115W, F150W, and F200W) bands reveal a large population of unresolved point-like sources across the field, the vast majority of which are globular clusters (GCs) or ultra-compact dwarfs. To the limits of our photometry, more than 10 000 such objects were measured, most of which are in the haloes of the five largest A2744 galaxies but which also include GCs around some satellite galaxies and throughout the intergalactic medium. The measured luminosity function covers almost 4 mag and follows a classic lognormal shape, though the GCLF turnover point still lies fainter than the photometric completeness limit. The colour index (F115W–F200W) in particular covers an intrinsic spread of 0.5 mag, clearly resolving the expected range of GC metallicities. The overall results are consistent with a large, normal GC population seen at a 3.5 Gyr earlier stage of dynamical evolution.

The presence or absence of star clusters in galaxies, and the properties of star cluster populations compared to their host galaxy properties, are important observables for validating models of cluster formation, galaxy formation, and galaxy assembly. In this work, we apply a Bayesian approach to fit two models to data from surveys of young clusters in star-forming galaxies. The first model is a logistic regression, which allows us to include galaxies which do not have any young clusters. The second model is a hurdle model, which includes galaxies with zero clusters and also incorporates information about the total mass in the cluster system. We investigate two predictors (star formation rate and total stellar mass in the galaxy) and look at clusters younger than 10 or 100 Myr. We find that in all cases, star formation rate is the better predictor for both the probability of hosting clusters and the total mass in the cluster system. We compare our results to similar models for old globular clusters, and conclude that star cluster formation was more abundant and more efficient at higher redshifts, likely because of the high gas content of galaxies at that time.