The THESAN-ZOOM project: Population III star formation continues until the end of reionization

Abstract: Population III (Pop III) stars are the first stars in the Universe, forming from pristine, metal-free gas and marking the end of the cosmic dark ages. Their formation rate is expected to sharply decline after redshift $z \approx 15$ due to metal enrichment from previous generations of stars. In this paper, we analyze 14 zoom-in simulations from the THESAN-ZOOM project, which evolves different haloes from the THESAN-1 cosmological box down to redshift $z=3$. The high mass resolution of up to $142 M_\odot$ per cell in the gas phase combined with a multiphase model of the interstellar medium (ISM), radiative transfer including Lyman-Werner radiation, dust physics, and a non-equilibrium chemistry network that tracks molecular hydrogen, allows for a realistic but still approximate description of Pop III star formation in pristine gas. Our results show that Pop III stars continue to form in low-mass haloes ranging from $10^6 M_\odot$ to $10^9 M_\odot$ until the end of reionization at around $z=5$. At this stage, photoevaporation suppresses further star formation in these minihaloes, which subsequently merge into larger central haloes. Hence, the remnants of Pop III stars primarily reside in the satellite galaxies of larger haloes at lower redshifts. While direct detection of Pop III stars remains elusive, these results hint that lingering primordial star formation could leave observable imprints or indirectly affect the properties of high-redshift galaxies. Explicit Pop III feedback and specialized initial mass function modelling within the THESAN-ZOOM framework would further help interpreting emerging constraints from the James Webb Space Telescope.