On the Origin of the High Star-Formation Efficiency in Massive Galaxies at Cosmic Dawn

Abstract

Motivated by the early excess of bright galaxies seen by \textit{JWST}, we run zoom-in cosmological simulations of a massive galaxy at Cosmic Dawn (MDG), in a halo of \(10^{11} M_\odot\) at \(z = 9\), using the hydro-gravitational code RAMSES at an effective resolution \(\sim 10~{\rm pc}\). We investigate physical mechanisms that enhance the star-formation efficiencies (SFEs) under the unique conditions of high gas density (\(\sim 3\times 10^3~{\rm cm^{-3}}\), \(\sim 10^4~M_\odot/{\rm pc^2}\)). 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 of a few \(10^9~M_\odot\) and high star formation rate of \(\sim 100~M_\odot/{\rm yr}\). The high gas density makes supernova (SN) feedback less effective at suppressing star formation, producing a relatively high local SFE \(\gtrsim 10\%\). The global SFE is dominated by feedback-driven outflows and is only weakly correlated with the local SFE. Photoionization heating can enhance the effects of SN feedback on the local SFE by making more SNe explode in diffuse environments, but the global SFE remains high even in our simulations with the strongest feedback. Intense accretion at Cosmic Dawn seeds strong turbulence, which reduces the local SFE for the same gas conditions due to turbulent pressure support. This prevents star-forming clouds from catastrophically collapsing, but this only weakly affects the global SFE. The star formation histories of our simulated galaxies are in the ballpark of the MDGs seen by \textit{JWST}, despite our limited resolution. They set the stage for future simulations which treat radiation self-consistently and use a higher effective resolution \(\sim 1~{\rm pc}\) which captures the physics of star-forming clouds.