Starbursts at Cosmic Dawn: Formation of Globular Clusters, Ultra-Faint Dwarfs, and Population III star clusters at z > 6

Abstract

In the standard model of cosmology ($\Lambda$CDM) the first stars, star clusters, and galaxies are expected to have formed in low-mass dark matter halos at high redshifts ($z \sim 6 - 30$). Attempts to predict the properties and abundances of these objects have mainly relied on numerically expensive cosmological simulations, which often lack the sub-parsec resolution needed to resolve compact star clusters and/or neglect potentially important stellar feedback processes. Motivated by this, I introduce Anaxagoras, a detailed analytical ab initio model of starbursts in low-mass halos. The model includes gas cooling, central gas accretion and disk formation, and stellar feedback from direct radiation pressure, Ly$\alpha$ scattering and IR photons, stellar winds, expanding H II regions, and (crudely) supernovae. I apply Anaxagoras to star formation at $z > 6$ in satellite halos of the Milky Way, as well as to Population III (Pop III) star formation in minihalos. For the Milky Way setup, hundreds of galaxies are predicted to form with luminosities, half-mass radii, mass-to-light ratios, and ages in good agreement with the observed local population of Ultra-Faint Dwarfs. Furthermore, at least $\sim 40$ old globular cluster candidates with initial stellar masses $10^5 - 10^6\,M_\odot$ are predicted to form at the centers of low-mass halos. Finally, if Pop III stars are not overly massive ($25\,M_\odot$), between $\sim 1 - 30$ stars could form per minihalo at $z > 20$, increasing to $\sim 10 - 500$ at $z < 15$ as Lyman-Werner feedback delays star formation until halos reach larger masses; if Pop III stars are more massive ($140\,M_\odot$), most minihalos form just a single star.