The <scp>thesan-zoom</scp> project: star formation efficiencies in high-redshift galaxies

ABSTRACT Recent James Webb Space Telescope observations hint at unexpectedly intense cosmic star formation in the early Universe, often attributed to enhanced star formation efficiencies (SFEs). Here, we analyse the SFE in thesan-zoom, a novel zoom-in radiation-hydrodynamic simulation campaign of high-redshift ($z \gtrsim 3$) galaxies employing a state-of-the-art galaxy formation model resolving the multiphase interstellar medium (ISM). The halo-scale SFE ($\epsilon ^{\ast }_{\rm halo}$) – 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 $M_{\rm halo} \gtrsim 10^{9.5}{\, \rm M_\odot }$. The power-law slope transitions from $\sim 2/3$ to $\sim 1/3$ as halo mass increases, which hints at a transition from energy-driven to momentum-driven outflow. $\epsilon ^{\ast }_{\rm halo}$ is a factor of $2\!-\!3$ larger than commonly assumed in empirical galaxy formation models at $M_{\rm halo} \lesssim 10^{11}{\, \rm M_\odot }$. On galactic (pkpc) scales, the Kennicutt–Schmidt relation of neutral gas is universal in thesan-zoom, following $\Sigma _{\rm SFR} \propto \Sigma _{\rm gas}^2$, indicative of a turbulent energy balance in the ISM maintained by stellar feedback. The rise of $\epsilon ^{\ast }_{\rm halo}$ 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 $\epsilon ^{\ast }_{\rm halo}$ with redshift is modest, it is sufficient to explain the large observed number density of UV-bright galaxies at $z \gtrsim 12$. However, reproducing the brightest sources at $M_{\rm UV} \lesssim -21$ may require extrapolating the SFE beyond the halo mass range covered by thesan-zoom.