Star Cluster Formation and Survival in the First Galaxies

Abstract

Using radiation-hydrodynamic cosmological simulations, we present a detailed ($0.1$ pc resolution), physically motivated portrait of a typical-mass dwarf galaxy before the epoch of reionization, resolving the formation and evolution of star clusters into individual $10{\mathrm{M}}_{\odot }$ star particles. In the rest-frame UV, the galaxy has an irregular morphology with no bulge or galactic disk, dominated by light emitted from numerous, compact, and gravitationally-bound star clusters. This is especially interesting in light of recent HST and JWST observations that -- aided by the magnifying power of gravitational lenses -- have imaged, at parsec-scale resolution, individual young star clusters in the process of forming in similar galaxies at $z>6$. Because of their low metallicities and high temperatures, star-forming gas clouds in this galaxy have densities $\sim 100$ times higher than typical giant molecular clouds; hence, their star formation efficiencies are high enough ($f_{\ast }\sim 10-70$ per cent) to produce a sizeable population of potential globular cluster progenitors but typically smaller (between a few $100-2\times 10^4{\mathrm{M}}_{\odot }$, sizes of $0.1-3$ pc) and of lower metallicities ($10^{-3.5}-10^{-2.5}{\mathrm{Z}}_{\odot }$). The initial mass function of the star-forming clouds is log-normal while the bound star cluster mass function is a power-law with a slope that depends mainly on $f_{\ast }$ but also on the temporal proximity to a major starburst. We find slopes between $-0.5$ and $-2.5$ depending on the assumed sub-grid $f_{\ast }$. Star formation is self-regulated on galactic scales; however, the multi-modal metallicity distribution of the star clusters and the fraction of stars locked into surviving bound star clusters depends on $f_{\ast }$.

20 pages, 15 figures, submitted to MNRAS. See https://fred144.github.io/research.html for movie renders