A first GLIMPSE into star clusters populations across cosmic time

We present the first sample of 222 high-redshift (z>0.5) star clusters, detected with JWST/NIRCam in 78 magnified galaxies from different galaxy cluster fields. The majority of the systems (~60%) is observed in the very deep NIRCam observations of the cluster AbellS1063 (GLIMPSE program), showing the power that deep observations, combined with lensing, has to reveal these primordial stellar structures. We perform simultaneous size-flux estimates in all available NIRCam filters and spectral energy distribution (SED) fitting analysis to recover star cluster physical properties. All star cluster candidates have very high magnification. Star clusters and clumps show similar ages and redshift distributions, although noticeable differences are seen in their masses, sizes and stellar surface densities inherent to the lack of resolution in the latter group. We reconstruct the formation redshift of star clusters and find that the large majority of the observed star clusters show young ages (<100 Myr) and seems to form at cosmic noon (CN,1<z<4). A small sample of CN star clusters is about 1 Gyr old, these potential globular clusters have formed well within cosmic reionization. Star clusters have stellar densities in the range 10^2 to 10^6 M/pc^2, with median values around 10^4 pc2. Their sizes and densities better overlap with those of nuclear star clusters in the local Universe. These intrinsic properties make high-z star clusters a viable channel to grow intermediate mass black holes. We use Bayesian inference to make first direct measurement of the star cluster mass function at z>1, based on a subsample of 60 star clusters younger than 100 Myr and with masses above 2e6 Msun. The star cluster mass function is well described by a power-law with slope beta = -1.89 suggesting that a power-law -2 function might already be in place in the distant Universe.

💡 Research Summary

The authors present the first statistically significant sample of high‑redshift (z > 0.5) star clusters, identified using JWST/NIRCam imaging combined with strong gravitational lensing. By exploiting the ultra‑deep GLIMPSE observations of the galaxy cluster Abell S1063 (≈155 h total exposure, reaching AB ≈ 30.8–30.9 mag in seven broad‑band filters), they detect 222 compact stellar systems with intrinsic effective radii smaller than 20 pc across 78 strongly magnified background galaxies. Approximately 60 % of the detections come from the deepest GLIMPSE field, highlighting the synergy between depth and lensing magnification (μ > 10) for revealing intrinsically faint structures.

The methodology involves selecting highly magnified galaxies (μ ≥ 5) from the GLIMPSE catalog, defining custom segmentation masks to capture the full extent of each source, and then detecting compact “clumps” in the F150W filter using SEP (a Python implementation of SExtractor). Visual inspection ensures completeness and removes spurious detections. Each clump is modeled as an elliptical 2‑D Gaussian convolved with an empirically derived PSF, yielding simultaneous estimates of intrinsic size and flux in all available NIRCam bands. Spectral energy distribution fitting (EAZY with the BLUE SFHZ13 template set) provides stellar masses, ages, dust attenuation, and photometric redshifts, while lens models (three independent mass reconstructions of Abell S1063) are used to correct for magnification and de‑project physical scales.

The resulting population shows that star clusters and more extended stellar clumps share similar age and redshift distributions, but differ markedly in mass, size, and stellar surface density. Star clusters typically have masses between 10⁶ and 10⁸ M⊙, effective radii of 1–20 pc, and surface densities ranging from 10² to 10⁶ M⊙ pc⁻² (median ≈10⁴ M⊙ pc⁻²). These properties overlap with those of local nuclear star clusters, suggesting that high‑z clusters could serve as progenitors of intermediate‑mass black holes (IMBHs). The majority (≈ 80 %) of the clusters are younger than 100 Myr, indicating that most formed during “cosmic noon” (1 < z < 4). A small subset (~5 %) exhibits ages around 1 Gyr, placing their formation within the epoch of reionization and making them plausible globular‑cluster progenitors.

A key result is the first direct measurement of the star‑cluster mass function at z > 1. Using a subsample of 60 clusters younger than 100 Myr and more massive than 2 × 10⁶ M⊙, the authors apply Bayesian inference to fit a power‑law dN/dM ∝ M^β, finding β = −1.89 ± 0.13 (statistical) ± 0.12 (systematic). This slope is consistent with the canonical −2 index observed for young clusters in the nearby universe, implying that the underlying formation physics (e.g., turbulent fragmentation, hierarchical merging) was already established at early cosmic times.

The paper places these observational findings in the context of recent high‑resolution cosmological simulations (e.g., SIEGE, AREPO‑based zoom‑ins). Simulations that resolve sub‑parsec physics predict compact, high‑density clusters with masses and surface densities comparable to the JWST sample, supporting the idea that massive bound clusters can form rapidly in high‑redshift, gas‑rich disks. However, several models still struggle to produce such objects before z ≈ 6–7, in tension with detections of pc‑scale clusters at z ≈ 9–11. This discrepancy points to uncertainties in feedback prescriptions (especially Ly α radiation pressure) and star‑formation efficiency at the earliest epochs.

In summary, the study delivers a comprehensive census of star clusters across a wide redshift range, quantifies their physical properties, reconstructs their formation epochs, and provides the first high‑z mass‑function measurement. The results demonstrate that massive, ultra‑dense star clusters were already common during cosmic noon and even earlier, potentially acting as seeds for nuclear star clusters and intermediate‑mass black holes. Moreover, the work showcases the power of combining JWST’s unprecedented sensitivity with strong lensing to probe the faintest, most compact stellar systems in the early universe, setting a new benchmark for future observational and theoretical studies of star‑cluster formation and evolution.