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

The early universe, particularly the epoch of reionization, is a period of intense star formation and galaxy evolution. Understanding the processes that shaped the first galaxies and their constituent stellar populations is crucial for comprehending the overall cosmic evolution. This research focuses on identifying and characterizing star clusters in a high-redshift galaxy, using gravitational lensing to magnify the faint objects and enabling detailed observations. The research question centers on whether compact, dense star clusters – potential progenitors of globular clusters – existed in the early universe and what their properties might reveal about the conditions and processes under which they formed. The purpose is to provide observational evidence of these early star clusters and to use their physical characteristics (mass, size, density, age) to constrain theoretical models of proto-globular cluster formation. The importance of this study lies in its potential to shed light on the early stages of galaxy assembly, the role of star clusters in reionization, and the formation of globular clusters in the Milky Way.

Previous studies have identified high-redshift galaxies and star-forming regions, but resolving individual star clusters at such distances has been challenging. Studies using gravitational lensing have shown promise in magnifying distant objects, allowing for more detailed observations. Several papers cited in the original article discuss the characteristics of star clusters in the local universe, in moderately lensed galaxies at redshift z = 10, and at lower redshifts (z = 6 and z = 2.7). These studies provide a basis for comparison with the newly observed clusters and offer insights into potential evolutionary paths. The existing literature highlights the need for further investigation into the formation and evolution of star clusters at high redshift to understand their contribution to early galaxy formation and reionization. The paper also references studies on stellar feedback, black hole formation in dense clusters, and the overall impact of early star clusters on galaxy evolution.

The researchers utilized data from the James Webb Space Telescope (JWST) to observe the Cosmic Gems arc (SPT0615-JD1), a gravitationally lensed galaxy at a redshift of z ≈ 10. Gravitational lensing significantly magnifies the light from the source galaxy, making it possible to detect and analyze individual star clusters within it. The analysis involved sophisticated image processing techniques to deconvolve the observed light profiles of the star clusters. The physical properties of the clusters, including their half-light radii (R_eff), stellar masses (M*), and stellar surface densities (Σ), were determined using modeling techniques with BAGPIPES and PROSPECTOR. These models incorporate various assumptions about star formation history (SFH), the initial mass function (IMF), and the presence of stellar binaries. The impact of lensing magnification uncertainties on the derived quantities was assessed through a careful error analysis using different lensing models (Lenstool and Glafic). Two independent methods were used to measure the intrinsic half-light radii, one by projecting cluster shapes and another through image plane measurements, providing cross-validation. The dynamical age of the clusters was also calculated to determine if they are gravitationally bound, using the framework where a star cluster is considered bound if its age is greater than its crossing time. The analysis compared the properties of the observed clusters to those of known star clusters in the local universe, including young massive clusters and globular clusters in the Milky Way.

The study identified five gravitationally bound star clusters within the Cosmic Gems arc. These clusters are remarkably compact, with intrinsic effective half-light radii (R_eff) close to 1 pc (within uncertainties). Their stellar surface densities are exceptionally high, reaching approximately 10^4 M☉ pc⁻², implying high stellar densities of about 10⁶ M☉ pc⁻³. The clusters' stellar masses range from approximately 1 to 7 solar masses. The derived dynamical ages suggest that these clusters are gravitationally bound and have ages consistent with their formation around 460 Myr after the Big Bang. The clusters exhibit physical properties consistent with those observed in lensed galaxies at z = 6 and z = 2.3, which strengthen the reliability of the estimated physical properties. The compactness of the clusters may be related to the leakage of hydrogen ionizing radiation from their natal clouds, potentially impacting cosmic evolution. The high stellar densities in these clusters could also significantly enhance stellar black hole mergers and contribute to the formation of intermediate-mass black holes. The observed properties of the Cosmic Gems clusters support theoretical predictions that suggest such very dense stellar clusters would form in low-metallicity and high-density gas environments. These findings point to the existence of dense, proto-globular clusters in the early universe.

The discovery of compact, dense, and gravitationally bound star clusters in a galaxy 460 million years after the Big Bang provides compelling observational evidence supporting models that predict the formation of such structures in the early universe. The high stellar surface densities and small sizes of these clusters, compared to local young massive clusters and Milky Way globular clusters, suggest unique formation conditions in the early universe. The finding that these systems are likely proto-globular clusters has important implications for understanding the formation and evolution of globular clusters in galaxies. This discovery contributes to understanding the role of star clusters in the reionization era, their potential influence on galaxy formation, and the possible connections between early star clusters and the enriched stellar populations observed in Milky Way globular clusters today. The analysis of dynamical evolution suggests that the proto-globular clusters are likely to experience expansion due to mass loss, relaxation, and external tidal forces. However, their initial compactness suggests they are likely to survive and potentially evolve into mature globular clusters.

This study presents compelling evidence for the existence of gravitationally bound, dense star clusters in a lensed galaxy only 460 million years after the Big Bang. These clusters exhibit unusual compactness and high stellar densities compared to local analogs, providing key insights into proto-globular cluster formation and evolution. The findings support theoretical predictions of early universe conditions, highlighting their significance for understanding galaxy formation, reionization, and the origins of globular clusters. Future research could explore additional lensed galaxies to better understand the prevalence of such systems and their diversity, further refine modeling of proto-globular cluster formation and evolution, and study their role in reionization and galaxy evolution through deeper spectroscopic observations.

The analysis relies on the accuracy of the gravitational lensing models used to correct for magnification effects. Uncertainties in lensing models could affect the derived physical properties of the star clusters. The sample size is limited to five clusters, making it difficult to draw definitive conclusions about the broader population of star clusters in early galaxies. The reliance on specific model assumptions about the SFH and IMF could influence the results. Furthermore, some clusters were not fully resolved limiting the precision of certain measurements, and the impact of stellar feedback is not fully quantified in the study.