Cosmic Dawn (CoDa): the First Radiation-Hydrodynamics Simulation of
  Reionization and Galaxy Formation in the Local Universe
  Cosmic reionization by starlight from early galaxies affected their
evolution, thereby impacting reionization, itself. Star formation suppression,
for example, may explain the observed underabundance of Local Group dwarfs
relative to N-body predictions for Cold Dark Matter. Reionization modelling
requires simulating volumes large enough [~(100Mpc)^3] to sample reionization
"patchiness", while resolving millions of galaxy sources above ~10^8 Msun ,
combining gravitational and gas dynamics with radiative transfer. Modelling the
Local Group requires initial cosmological density fluctuations pre-selected to
form the well-known structures of the local universe today. Cosmic Dawn
("CoDa") is the first such fully-coupled, radiation-hydrodynamics simulation of
reionization of the local universe. Our new hybrid CPU-GPU code,
RAMSES-CUDATON, performs hundreds of radiative transfer and ionization
rate-solver timesteps on the GPUs for each hydro-gravity timestep on the CPUs.
CoDa simulated (91Mpc)^3 with 4096^3 particles and cells, to redshift 4.23, on
ORNL supercomputer Titan, utilizing 8192 cores and 8192 GPUs. Global
reionization ended slightly later than observed. However, a simple temporal
rescaling which brings the evolution of ionized fraction into agreement with
observations also reconciles ionizing flux density, cosmic star formation
history, CMB electron scattering optical depth and galaxy UV luminosity
function with their observed values. Photoionization heating suppressed the
star formation of haloes below ~2 x 10^9 Msun , decreasing the abun- dance of
faint galaxies around MAB_1600 = [-10,-12]. For most of reionization, star
formation was dominated by haloes between 10^10 - 10^11 Msun , so low-mass halo
suppression was not reflected by a distinct feature in the global star
formation history. (Abridged)
