The Inhomogeneous Reionization Times of Present-day Galaxies

Today’s galaxies experienced cosmic reionization at different times in different locations. For the first time, reionization (50% ionized) redshifts, z_R , at the location of their progenitors are derived from new, fully coupled radiation-hydrodynamics simulation of galaxy formation and reionization at z > 6, matched to N-body simulation to z = 0. Constrained initial conditions were chosen to form the well-known structures of the local universe, including the Local Group and Virgo, in a (91 Mpc)³ volume large enough to model both global and local reionization. Reionization simulation CoDa I-AMR, by CPU-GPU code EMMA, used (2048)³ particles and (2048)³ initial cells, adaptively refined, while N-body simulation CoDa I-DM2048, by Gadget2, used (2048)³ particles, to find reionization times for all galaxies at z = 0 with masses M(z = 0) ≥ 10⁸M_⊙. Galaxies with 

 reionized earlier than the universe as a whole, by up to ~500 Myr, with significant scatter. For Milky Way–like galaxies, z_R ranged from 8 to 15. Galaxies with 

 typically reionized as late or later than globally averaged 50% reionization at 

, in neighborhoods where reionization was completed by external radiation. The spread of reionization times within galaxies was sometimes as large as the galaxy-to-galaxy scatter. The Milky Way and M31 reionized earlier than global reionization but later than typical for their mass, neither dominated by external radiation. Their most-massive progenitors at z > 6 had z_R  =9.8 (MW) and 11 (M31), while their total masses had z_R  = 8.2 (both).