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 $M(z=0)\ge 10^8 M_\odot$. Galaxies with $M(z=0) \gtrsim
10^{11} M_\odot$ reionized earlier than the universe as a whole, by up to
$\sim$ 500 Myrs, with significant scatter. For Milky-Way-like galaxies, $z_R$
ranged from 8 to 15. Galaxies with $M(z=0) \lesssim 10^{11} M_\odot$ typically
reionized as late or later than globally-averaged $50\%$ reionization at
$\langle z_R\rangle =7.8$, 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).
