Cosmic Wallflowers: How Lonely Star Clusters Bloomed in the Early Universe
In this paper, van Donkelaar and collaborators investigate a surprising population of extremely compact star clusters forming very early in cosmic history, at redshifts (z > 7), less than 800 million years after the Big Bang. Recent discoveries with the James Webb Space Telescope (JWST) revealed tiny, dense stellar systems in strongly lensed galaxies, raising new questions about how and where such clusters form. Rather than focusing on star formation inside galactic discs, the authors explore a quieter and less obvious environment: the circumgalactic medium (CGM), the gas-rich region surrounding young galaxies. Their goal is to understand whether clusters forming in these outskirts could be connected to present-day globular clusters and the seeds of massive black holes.
Simulation Framework and Cluster Identification
To address this problem, the authors use the high-resolution cosmological hydrodynamical simulation MassiveBlackPS, which follows the evolution of gas, stars, and dark matter with spatial resolution down to a few parsecs. From this simulation, they identify 55 compact stellar systems forming outside galactic discs but still within the virial radius of a massive halo at (z ~ 7.6). Strict selection criteria ensure that these objects are genuine star clusters rather than small galaxies: they are compact (with stellar half-mass radii below about 15 parsecs), relatively low-mass, and strongly baryon-dominated, meaning they contain negligible amounts of dark matter.
Basic Properties of the Cosmic Wallflowers
The identified clusters exhibit extreme properties. Many reach stellar surface densities closely matching the compact clusters recently observed by JWST in systems such as the Cosmic Gems Arc. Most clusters still retain significant gas fractions, indicating that they are observed at an early formation stage rather than as fully evolved, gas-free systems. Their star formation rates span a wide range, from actively star-forming clusters comparable to observed high-redshift systems to others where star formation has already dropped to very low levels, suggesting a diversity of evolutionary states.
Structural Properties and Metallicity
The structural analysis shows that these clusters are far denser than typical star clusters in the local Universe and overlap in size and density with the most compact systems known at high redshift. The clusters also display a broad range of stellar metallicities, from very metal-poor to near-solar values. This spread is much larger than that seen for clusters forming inside galactic discs, reflecting the chemically diverse environments of the CGM. Notably, the densest clusters tend to have higher metallicities, because metal-rich gas cools more efficiently, allowing it to collapse to higher densities before stellar feedback halts further contraction.
Formation Through Filament Fragmentation
A central result of the paper is that these clusters form through gravitational fragmentation of gas filaments in the CGM. As cold gas streams flow along filaments toward galaxies, local regions can become unstable and collapse under their own gravity. By comparing the gas mass near each cluster to a critical mass for gravitational instability and by tracking how gas temperature and density evolve shortly before cluster formation, the authors show that most clusters form in regions that cool rapidly and become dense enough to collapse. This filament-fragmentation pathway differs from classic disc fragmentation and highlights the importance of off-disc environments in early star formation.
Implications for Black Hole Seeds
Because many clusters reach extremely high stellar densities, the authors explore whether they could host the formation of intermediate-mass black holes (IMBHs). In such dense environments, runaway stellar collisions can produce very massive stars that collapse into black holes with masses of thousands of solar masses. A subset of the clusters has both the high densities and sufficiently low metallicities required for this process. While not all clusters are expected to form IMBHs or survive intact, some may migrate inward and contribute to the early growth of central black holes, while others could leave behind wandering black holes in galactic halos.
Conclusions and Broader Context
The study concludes that star cluster formation in the early Universe was not limited to galactic discs. Instead, the CGM and its filamentary structure provided fertile ground for forming dense, compact stellar systems in relative isolation. These “cosmic wallflowers” offer a natural link between high-redshift JWST observations, the possible origins of present-day globular clusters, and the formation of black hole seeds. By highlighting the role of quiet, off-disc environments, the paper expands our understanding of how the first generations of star clusters shaped early galaxy evolution.
Source: van Donkelaar
