Patchy Reionization Delays Quenching in Isolated Ultra-faint Dwarf Galaxies

Abstract

The spatially inhomogeneous nature of cosmic reionization is directly encoded in the quenching timescales of ultra-faint dwarf galaxies (UFDs). While most cosmological simulations employ a spatially uniform UV background (UVB) for computational efficiency, such homogeneous treatment may oversimplify the physical impact of reionization on UFDs, whose star formation histories (SFHs) are highly sensitive to local environments. Under these idealized, instantaneous UVB frameworks, star formation in UFDs is typically truncated abruptly at the onset of reionization, creating a tension with the extended and diverse SFHs observed in Local Group UFDs. To address this discrepancy, we perform high-resolution cosmological zoom-in simulations to contrast two distinct reionization models: a conventional uniform UVB and an on-the-fly radiative transfer (RT)-based patchy reionization model. We find that RT-based reionization delays star-formation quenching by ~440 Myr on average, a result consistent with recent observational constraints. However, this extended star formation does not necessarily leave a discernible imprint on the stellar metallicity distribution, indicating that the delay--while physically significant for gas dynamics--is still insufficient to drive substantial chemical enrichment. Our results emphasize the necessity of more realistic, patchy reionization modeling to explain the observed diversity of the Local Group population.