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 transitions from ∼2/3 to ∼1/3 as halo mass increases, which hints at a transition from energy-driven to momentum-driven outflow. ε ₕₐₗₒ is a factor of 2 − 3 larger than commonly assumed in empirical galaxy formation models at Mₕₐₗₒ 10¹¹\, M_. On galactic (pkpc) scales, the Kennicutt–Schmidt (KS) relation of neutral gas is universal in thesan-zoom, following Σ SFR ∝ Σ gas², 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. These results are robust against variations in numerical resolution and star-formation and feedback models, depending mainly on the total feedback momentum budget. Although the increase in ε ₕₐₗₒ with redshift is 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 covered by thesan-zoom.