Abstract The “near–far” approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ( z ≳ 6), beyond the current James Webb Space Telescope detection limits ( M UV ≳ −15). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to ∼50% of the total stellar-mass budget of the proto-MW/LG at z = 6−9. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of ∼2–3 and reduces the halo-to-halo variation in the slope by ∼20%–40%. This enables robust constraints down to at least the resolution limit of the simulations, near M ⋆ ∼ 10 5 M ⊙ or M UV ∼ −10 at z ≳ 6. We also show that “fossil-record” reconstructions—which assume each present-day system descends from a single reionization-era progenitor—are sensitive to the stellar-mass/UV-magnitude thresholds, which introduces bias in the inferred slopes at the low-mass/faint end. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with M UV ≲ −15 to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction (∼50%) of streams with M ⋆ ≳ 10 6 M ⊙ at z = 0 should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.
Kundu et al. (Fri,) studied this question.