The Inhomogeneous Reionization Times of Present-day Galaxies

Dominique Aubert, Nicolas Deparis, Pierre Ocvirk, Paul R. Shapiro, Ilian T. Iliev, Gustavo Yepes, Stefan Gottlöber, Yehuda Hoffman, Romain Teyssier

Research output: Contribution to journalArticlepeer-review

30 Scopus citations


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) ≥ 108 Mo. 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).

Original languageEnglish (US)
Article numberL22
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Apr 1 2018
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • dark ages
  • first stars
  • galaxies
  • high-redshift
  • methods
  • numerical
  • reionization


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