Reionization in HESTIA: Studying reionization in the LG through zoom simulations

Abstract

While cosmic reionization has been broadly constrained by global observables, the interplay between internal sources (Milky Way, M31, and their satellites) and external ionization fronts remains poorly understood in a realistic Local Group (LG) context. To address this issue, we perform radiative transfer post-processing on the original HESTIA LG constrained simulation. We calibrate our source models using a uniform 1024^3 particle, dark-matter only, HESTIA simulation coupled with a subgrid collapse-fraction model to match the global reionization observables. These source models are then applied to the HESTIA zoom-in simulations, which consist of a 4096^3 particle effective resolution in the zoom region centered on the Milky Way (MW) and M31 haloes, which resolves haloes down to 10^8 solar masses. We find that in all scenarios, reionization within the LG proceeds in an inside-out manner with the progenitors of the MW and M31 having 50 percent of their material ionized by z ~ 9-8.6, significantly earlier than the global midpoint at z ~ 7-7.7, noting that external fronts from large-scale structure play a negligible role, even under the most permissive feedback model. We further show that present-day satellite galaxies exhibit only a weak correlation between their reionization redshift and their present-day radial distance from their host halo, with somewhat tighter trends around M31 than the MW. Finally, we find that satellites which assembled before reionization are systematically more massive today, suggesting that the oldest stellar populations preferentially reside in the most massive subhaloes.