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

This study investigates whether compact stellar systems observed in the strongly lensed z ≈ 10 Cosmic Gems arc (SPT0615-JD1) are gravitationally bound star clusters only ~460 Myr after the Big Bang. The context is the role of dense, young clusters in early galaxy evolution and reionization. By measuring sizes, masses, and surface densities and comparing dynamical timescales to ages, the authors test the hypothesis that these systems are bona fide proto-globular clusters, probing cluster formation mechanisms in the early Universe.

The work builds on recent JWST observations of high-redshift galaxies and star clusters, including lensed systems at z ≈ 6 and z ≈ 2.7, and comparisons to Milky Way globular clusters and young massive clusters in nearby galaxies. Theoretical frameworks considered include feedback-free starburst models, the role of compact starbursts in ionizing photon leakage, and dynamical evolution processes (mass loss from stellar evolution, two-body relaxation including stellar-mass black holes, and tidal effects) shaping present-day globular clusters. Prior studies suggest that extreme conditions (low metallicity, high gas density) can yield very dense clusters with high star-formation efficiencies, potentially enhancing rates of black hole mergers and formation of very massive or even supermassive stars, with implications for reionization and multiple stellar populations in globular clusters.

- Observations and lensing: The team studies compact sources within the strongly lensed Cosmic Gems arc (SPT0615-JD1). They use a reference lensing model (Lenstool) and compare with alternative models (e.g., Glafic) to derive magnification, intrinsic sizes, and surface densities. Intrinsic sizes are obtained by deconvolving observed half-light radii and correcting for seeing/PSF and lensing magnification. Forward modeling and projecting cluster shapes between source and image planes provide independent intrinsic size estimates. - Size measurements: Observed effective half-light radii Re are measured; for marginally resolved or unresolved sources, upper limits are set by the PSF half-maximum (F150W PSF ≈ 0.05″ ≈ 1.25 pc). Intrinsic Re are derived at the clusters’ locations using the lens model. Independent R_e estimates (R_ut) are also obtained via source-plane to image-plane projection to validate sizes. - Photometric/SFH modeling: Stellar masses, ages, extinction, and metallicities are derived using SED-fitting codes BAGPIPES and Prospector. Multiple star-formation history assumptions (including simple bursts appropriate for compact systems), different high-mass IMF limits, and inclusion of binary stars are tested to assess robustness. Median stellar masses are taken from BAGPIPES reference fits. Uncertainties correspond to 68% confidence intervals. - Derived quantities: Using magnifications, the authors compute stellar surface densities Σ and dynamical age parameters. A cluster is considered gravitationally bound if its age exceeds the crossing time (virial equilibrium assumed), i.e., λ = age / t_cross > 1. Magnification uncertainty evaluations are described in Methods; error bars shown do not include magnification uncertainties, but tests indicate conclusions are insensitive to them. - Spatial context: The clusters reside within a very compact region in the source plane; observed projected separations and scale estimates are used to contextualize size and density measurements.

- Detection of five compact star clusters within the z ≈ 10 Cosmic Gems arc with intrinsic half-light radii close to ~1 pc. Alternative lens models yield consistent ranges: ~0.3–0.9 pc (Lenstool variant) and ~0.3–1.2 pc (Glafic). - Some sources are marginally resolved; others are consistent with being unresolved, with upper limits set by the PSF (≈1.25 pc). - Stellar surface densities are extremely high, Σ ≈ 10^4 M⊙ pc−2, indicative of very dense stellar systems. - SED fits using BAGPIPES and Prospector, across varying SFH, IMF upper-mass limits, and binary assumptions, yield broadly consistent ages, masses, extinction, and metallicities for the clusters. - Dynamical age parameters (age to crossing-time ratios) are significantly greater than unity, demonstrating the systems are gravitationally bound despite measurement and lensing uncertainties. - The clusters are more compact and denser than typical young star clusters in the local Universe and than present-day Milky Way globular clusters, aligning with expectations for proto-globular clusters at early times. - Results are consistent with other lensed cluster detections at z ≈ 6 and z ≈ 2.7, reinforcing the presence of bound massive clusters across cosmic time.

The findings directly address whether very early compact stellar systems are gravitationally bound. The combination of ~pc-scale sizes, Σ ≈ 10^4 M⊙ pc−2, and dynamical age parameters >1 indicates these are bound clusters only ~460 Myr after the Big Bang, supporting scenarios where proto-globular clusters form under extreme conditions in the reionization era. Their compactness may facilitate leakage of ionizing photons, contributing to the ionizing budget of high-z galaxies. High densities and low metallicities favor high star-formation efficiencies and may promote dynamical processes such as runaway collisions, formation of very massive stars, and enhanced stellar black hole mergers, potentially leading to intermediate-mass black holes. The offset relative to local young clusters can be explained by more extreme physical conditions at high redshift and by dynamical evolution over a Hubble time transforming proto-clusters into present-day globular clusters through stellar mass loss, relaxation (including SBH subsystems), and tidal effects. The robustness of the conclusions against lensing-model variations and magnification uncertainties strengthens the case that these are bona fide proto-globular clusters.

This study identifies and characterizes five extremely compact, dense, gravitationally bound star clusters in a strongly lensed z ≈ 10 galaxy, providing evidence for proto-globular cluster formation only 460 Myr after the Big Bang. Intrinsic sizes (~1 pc), very high stellar surface densities (~10^4 M⊙ pc−2), and dynamical age parameters >1 indicate bound systems, with results robust to lensing-model variations and SED-model assumptions. These observations inform models of early cluster formation, the ionizing photon budget in reionization-era galaxies, and pathways to black hole growth. Future work should expand samples of lensed high-z clusters, obtain spectroscopic constraints to refine ages/metallicities, reduce magnification uncertainties with improved lens models, and follow the dynamical evolution of such systems in simulations to connect them to present-day globular clusters.

- Lensing magnification uncertainties propagate into size, mass, and surface density estimates; while tested, formal error bars shown do not include magnification uncertainties. - Some clusters are unresolved or only marginally resolved, yielding upper limits on sizes and affecting derived densities. - Physical parameter inferences depend on SED-fitting assumptions (SFH, IMF upper-mass cutoff, binary treatment) and dust/metallicity priors. - Small sample size within a single lensed system may limit generalizability. - The long-term survival of such clusters is uncertain and not directly constrained by the data.