Star Cluster Formation and Survival in the First Galaxies

TL;DR

This study uses high-resolution cosmological simulations to explore star cluster formation in a typical early dwarf galaxy, revealing properties consistent with recent high-redshift observations and identifying key factors influencing cluster survival.

Contribution

It provides a detailed, physically motivated simulation of star cluster formation in a first galaxy, linking cluster properties to star formation efficiency and galaxy evolution.

Findings

Star clusters have masses between a few hundred and 2×10^4 solar masses.

Star formation efficiencies range from 10% to 70%.

Cluster metallicities are very low, between 10^-3.5 and 10^-2.5 Z_sun.

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 ($f_*\sim10-70$ per cent) to produce a sizeable population of potential globular cluster progenitors but typically smaller (between a few $100\:-\: 2\times10^4\:\mathrm{M_{\odot}}$, 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 $f_*$ 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 $f_*$. 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 $f_*$.