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

Abstract

We model the formation of star clusters in a dwarf galaxy progenitor during the first $700\mathrm{Myr}$ 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 $\sim 10^{2-4}{\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$, radii $\lesssim 3\mathrm{pc}$) resembling those observed by the James Webb Space Telescope (JWST) in strongly lensed galaxies. Star clusters follow a flat power-law mass function $\mathrm{d}N/\mathrm{d}\log M\propto M^{\hat{I}}“$ with slope $\hat{I}“\sim -0.4$. The most massive star clusters ($10^{4-5}{\mathrm{M}}_{\odot }$) 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: $-1.95<\log (Z/{\mathrm{Z}}_{\odot })<-1.50$ at lower SFEs ($2-20\%$). At $z\sim 8.7$, a nuclear star cluster (NSC) is seeded, growing $83\%$ of its mass ($2.4\times 10^5{\mathrm{M}}_{\odot }$, $20\%$ 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 $\text{little red dots}$ recently discovered by JWST at $z\gtrsim 5$

Published in the Open Journal of Astrophysics