2026/01/27 by Michelle C. Jecmen, John Chisholm, Hakim Atek +23
#astro-ph.GA
As observations have yet to constrain the ionizing properties of the faintest (M\rm 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\rm esc) for such faint galaxies at high-redshift. Leveraging ultra-deep JWST/NIRCam GLIMPSE imaging of strong lensing field Abell S1063, we estimate UV continuum slopes of 555 galaxies at z > 6 with absolute magnitudes down to M\rm UV ≃ -12.5. We find a modest evolution of β with redshift and a flattening in the β-M\rm UV relation such that galaxies fainter than M\rm UV ∼ -16.5 no longer exhibit the bluest UV slopes. The 138 ultra-faint galaxies with M\rm UV > -16 are a diverse population encompassing dusty (30%), old (15%), and low-mass (50%) galaxies. We apply the empirical β-f\rm esc relation from local Lyman continuum leakers, finding the mean f\rm esc peaks at ∼ 20% at M\rm UV=-16.5 and declines towards fainter galaxies, while remaining consistent with f\rm esc = 14% within 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 M\rm UV between -18 and -14 supplied ∼ 60% of the ionizing photons to cosmic reionization, while the lower f\rm esc of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.