Seeding Cores: A Pathway for Nuclear Star Clusters from Bound Star Clusters in the First Billion Years

Fred Angelo Batan Garcia, Massimo Ricotti, Kazuyuki Sugimura

First listed 2025-03-11 | Last updated 2025-10-01

Abstract

We model the formation of star clusters in a dwarf galaxy progenitor during the first $700 ~{\rm Myr}$ of cosmic history using a cosmological radiation-hydrodynamic simulation with a sub-grid star formation efficiency (SFE) model calibrated from AU-scale radiation-MHD simulations of molecular clouds with varying mass, density, and metallicity. In comparison to a constant SFE model, our model yields more bursty star formation, a more abundant massive star cluster population, and overall a higher stellar mass. Clouds reach SFEs up to $80\%$, forming bound star clusters (densities $\sim10^{2-4} ~{\rm M_\odot\:pc^{-2}}$, radii $\lesssim 3~{\rm pc}$) resembling those observed by the James Webb Space Telescope (JWST) in strongly lensed galaxies. Star clusters follow a flat power-law mass function ${\rm d}N/{\rm d}\log M \propto M^Γ$ with slope $Γ\sim -0.4$. The most massive star clusters ($10^{4-5} ~{\rm M_\odot}$) grow through mergers and have metallicity spreads of $0.05 - 0.1$ dex that roughly scale with mass. The second burst of star formation produce loosely bound star clusters with higher metallicities: $-1.95 < \log(Z/{\rm Z_\odot}) < -1.50$ at lower SFEs ($2 - 20\%$). At $z \sim 8.7$, a nuclear star cluster (NSC) is seeded, growing $83\%$ of its mass ($ 2.4 \times 10^5 ~{\rm M_\odot}$, $20\%$ of the galaxy's stellar mass) through mergers with pre-existing clusters and the rest through in-situ star formation. The early formation of NSCs has interesting implications for seeding supermassive black holes and the population of $\textit{little red dots}$ recently discovered by JWST at $z \gtrsim 5$

Short digest

Cosmological RHD zoom-ins of a dwarf progenitor with a variable, cloud-scale SFE (calibrated on AU-scale RMHD) produce burstier star formation, a richer massive cluster population, and higher total stellar mass than constant-SFE runs. Dense clouds reach up to ~80% SFE, yielding bound star clusters (Σ~10^2–10^4 M_sun pc^-2, r<=3 pc) with a flat mass function (dN/dlogM ∝ M^Γ, Γ≈−0.4); the most massive (10^4–10^5 M_sun) assemble via mergers and show 0.05–0.1 dex metallicity spreads. A nuclear star cluster is seeded by z≈8.7 and grows to ~2.4×10^5 M_sun, with 83% of its mass built through cluster mergers, reaching ~20% of the galaxy’s stellar mass. This early, merger-driven NSC pathway provides a natural route to SMBH seeding and links to the compact “little red dots” population at z≳5.

Key figures to inspect

• Figure 1: Inspect the zoom-in panels to see the compact, bound clusters after starburst (a); verify sizes (<=3 pc) and relative brightness, and note the most massive cluster contributing ~10% of the galaxy’s stellar mass.
• Figure 2: Compare SFR histories across SFE prescriptions, focusing on burstiness, duty cycles, and the labeled starbursts (a–h); use this timeline to place when the NSC is seeded relative to the main bursts.
• Figure 3: Read SFE versus cloud mass, colored by surface density; confirm the early, dense clouds reaching ~80% SFE and contrast with the 35%/70% constant-SFE guides to see why bound clusters are more prevalent.
• Figure 4: Track metallicity versus cloud mass across epochs and relate to the second burst producing higher-metallicity, loosely bound clusters (2–20% SFE); check the bottom-panel mass function colored by SFE for the ~Γ=−0.4 slope.