A GLIMPSE into the UV Continuum Slopes of the Faintest Galaxies in the Epoch of Reionization

Abstract As observations have yet to constrain the ionizing properties of the faintest ( M UV ≳ −16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope ( β ) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons ( f esc ) for such faint galaxies at high redshift. Leveraging ultradeep JWST/NIRCam GLIMPSE imaging of the strong lensing field Abell S1063, we estimate the UV continuum slopes of 553 galaxies at z > 6 with absolute magnitudes down to M UV ≃ −12.5. We find a modest evolution of β with redshift and a flattening in the β – M UV relation such that galaxies fainter than M UV ∼ −16.5 no longer exhibit the bluest UV slopes. The 136 ultrafaint galaxies with M UV > −16 are a diverse population encompassing dusty (30%), old (15%), and low-mass (50%) galaxies. We apply the empirical β – f esc relation from local Lyman continuum leakers, finding the mean f esc peaks at ∼20% at M UV = −16.5 and declines towards fainter galaxies, while remaining consistent with f esc = 14% within the uncertainties, in agreement with recent radiative transfer simulations. Incorporating GLIMPSE constraints on the UV luminosity function, ionizing photon production efficiency, and escape fractions produces a reionization history consistent with independent observational constraints. Our results indicate galaxies with an M UV between −18 and −14 supplied ∼60% of the ionizing photons to cosmic reionization, while the lower f esc of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.