The THESAN-ZOOM project: Star-formation efficiencies in high-redshift galaxies

Recent JWST observations hint at unexpectedly intense cosmic star-formation in the early Universe, often attributed to enhanced star-formation efficiencies (SFEs). Here, we analyze the SFE in THESAN-ZOOM, a novel zoom-in radiation-hydrodynamic simulation campaign of high-redshift (z 3) galaxies employing a state-of-the-art galaxy formation model resolving the multiphase interstellar medium (ISM). The halo-scale SFE (ε^ₕₐₗₒ) - the fraction of baryons accreted by a halo that are converted to stars - follows a double power-law dependence on halo mass, with a mild redshift evolution above Mₕₐₗₒ 109.5\, M_. The power-law slope is roughly $1/3$ at large halo masses, consistent with expectations when gas outflows are momentum-driven. At lower masses, the slope is roughly $2/3$ and is more aligned with the energy-driven outflow scenario. ε^ₕₐₗₒ is a factor of $2-3$ larger than commonly assumed in empirical galaxy-formation models at Mₕₐₗₒ 10¹¹\, M_. On galactic (kpc) scales, the Kennicutt-Schmidt (KS) relation of neutral gas is universal in THESAN-ZOOM, following ΣSFR ∝ Σgas², indicative of a turbulent energy balance in the ISM maintained by stellar feedback. The rise of ε^ₕₐₗₒ with halo mass can be traced primarily to increasing gas surface densities in massive galaxies, while the underlying KS relation and neutral, star-forming gas fraction remain unchanged. Although the increase in ε^ₕₐₗₒ with redshift is relatively modest, it is sufficient to explain the large observed number density of UV-bright galaxies at z 12. However, reproducing the brightest sources at MUV -21 may require extrapolating the SFE beyond the halo mass range directly covered by THESAN-ZOOM.