Reionization in <scp>hestia</scp> : 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 (MW), 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 centred on the MW and M31 haloes, which resolves haloes down to $10^8$ M$_\odot$. 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 per cent of their material ionized by $z\approx 9 \!-\! 8.6$, significantly earlier than the global mid-point at $z\approx 7 \!-\! 7.7$, 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 present-day satellites whose assembly preceded the reionization of most of their $z=0$ material are systematically more massive today, suggesting that the oldest stellar populations preferentially reside in the most massive $z=0$ subhaloes.