  Recent observations with the James Webb Space Telescope (JWST) have uncovered
unexpectedly high cosmic star formation activity in the early Universe, mere
hundreds of millions of years after the Big Bang. These observations are often
understood to reflect an evolutionary shift in star formation efficiency (SFE)
caused by changing galactic conditions during these early epochs. We present
FIREbox-HR, a high-resolution, cosmological hydrodynamical simulation from the
Feedback in Realistic Environments project, which offers insights into the SFE
of galaxies during the first billion years of cosmic time. FIREbox-HR
re-simulates the cosmic volume (L = 22.1 cMpc) of the original FIREbox run with
eight times higher mass resolution (m_b ~ 7800 M_sun), but with identical
physics, down to z ~ 6. FIREbox-HR predicts ultraviolet (UV) luminosity
functions in good agreement with available observational data. The simulation
also successfully reproduces the observed cosmic UV luminosity density at z ~ 6
- 14, demonstrating that relatively high star formation activity in the early
Universe is a natural outcome of the baryonic processes encoded in the FIRE-2
model. According to FIREbox-HR, the SFE - halo mass relation for intermediate
mass halos (M_halo ~ 10^9 - 10^11 M_sun) does not significantly evolve with
redshift and is only weakly mass-dependent. These properties of the SFE - halo
mass relation lead to a larger contribution from lower mass halos at higher z,
driving the gradual evolution of the observed cosmic UV luminosity density. A
theoretical model based on the SFE - halo mass relation inferred from
FIREbox-HR allows us to explore implications for galaxy evolution. Future
observations of UV faint galaxies at z > 12 will provide an opportunity to
further test these predictions and deepen our understanding of star formation
during Cosmic Dawn.
