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 ) \geq 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 ) .