The THESAN-ZOOM project: Star formation efficiency from giant molecular clouds to galactic scale in high-redshift starbursts

Star formation in galaxies is inherently complex, involving the interplay of physical processes over a hierarchy of spatial scales. In this work, we investigate the connection between global (galaxy-scale) and local (cloud-scale) star formation efficiencies (SFEs) at high redshifts ($z\gtrsim 3$), using the state-of-the-art cosmological zoom-in simulation suite THESAN-ZOOM. We find that the galaxy-scale average SFE, $\langle ε^{\rm gal}_{\rm ff} \rangle$, scales with $M_{\rm halo}^{1/3}\,(1+z)^{1/2} \sim V_{\rm vir}$, consistent with expectations from feedback-regulated models. On cloud scales, we identify giant molecular clouds (GMCs) in a broad sample of high-redshift starbursts spanning a wide range of halo masses and redshifts. Star formation in these systems is predominantly hosted by filamentary GMCs embedded in a dense and highly turbulent interstellar medium (ISM). GMCs exhibit remarkably universal properties, including mass function, size, turbulence, and surface density, regardless of the environment in which they are identified. The global gas depletion time (and the Kennicutt-Schmidt relation) is determined by the GMC mass fraction in the ISM, while the cloud-scale SFE shows little variation. In particular, we find a nearly constant gas surface density of $Σ_{\rm GMC} \approx 70\,{\rm M}_{\odot}\,{\rm pc}^{-2}$ across different host galaxies. Nevertheless, we identify two regimes where phases with high SFE can arise. First, stars may form efficiently in the shock fronts generated by feedback from a preceding starburst. Second, the increasing background dark matter surface density with redshift may contribute to the gravitational potential of clouds at $z \gtrsim 8$ and confine them in high-SFE phases over extended periods.