On the origin of the high star formation efficiency in massive galaxies at Cosmic Dawn

Motivated by the early excess of bright galaxies seen by JWST, we run zoom-in cosmological simulations of a massive galaxy at Cosmic Dawn, in a halo of 10¹¹M_☉ at z = 9, using the hydro-gravitational code RAMSES at an effective resolution ∼ 10 pc. We investigate physical mechanisms that enhance the star formation efficiencies (SFEs) at the high gas densities of the star-forming regions in this galaxy (∼ 3 × 10³ cm⁻³, ∼ 10⁴ M_☉ pc⁻²). Our fiducial star formation recipe uses a physically motivated, turbulence-based, multi-freefall model, avoiding ad hoc extrapolation from lower redshifts. By z = 9, our simulated galaxy is a clumpy, thick, rotating disc with a high stellar mass ∼ 3 × 10⁹ M_☉ and high star formation rate ∼ 50 M_☉ yr⁻¹. The high gas density makes supernova (SN) feedback less efficient, producing a high local SFE ≳ 10 per cent. The global SFE is set by feedback-driven outflows and only weakly correlated with the local SFE. Photoionization heating makes SN feedback more efficient, but the integrated SFE always remains high. Intense accretion at Cosmic Dawn seeds turbulence that reduces local SFE, but this only weakly affects the global SFE. The star formation histories of our simulated galaxies are similar to observed massive galaxies at Cosmic Dawn, despite our limited resolution. We set the stage for future simulations which treat radiation self-consistently and use a higher effective resolution ∼ 1 pc that captures the physics of star-forming clouds.