Abstract
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)
Highlights
The epoch of reionization resulted from the escape of ionizing radiation from the first generation of star-forming galaxies into the otherwise cold, neutral gas of the primordial intergalactic medium (‘IGM’)
High-redshift galaxies and quasars have been observed, for example, which constrain the evolution of the IGM opacity to H Lymanα resonance scattering, the mean ionizing flux density, the UV luminosity function of galaxies, and the cosmic star formation history, while observations of the cosmic microwave background (‘CMB’) constrain the mean elecron scattering optical depth integrated through the entire IGM over time, back to the epoch of recombination, and give some information about the evolution of the mean ionized fraction
We emphasize the radiative-hydrodynamic processes, since it is the main novelty of this simulation, and finish by discussing the role of various halo mass ranges in the overall reionization process
Summary
The epoch of reionization (hereafter, ‘EoR’) resulted from the escape of ionizing radiation from the first generation of star-forming galaxies into the otherwise cold, neutral gas of the primordial intergalactic medium (‘IGM’). This radiation created intergalactic H II regions of ever-increasing size, surrounding the galaxies that created them, leading eventually to the complete overlap of neighbouring H II regions and the end of the EoR within the first billion years after the big bang. The cosmic 21-cm background from the evolving patchwork of intergalactic H I regions, not yet reionized, is a prime example, which is an important sciencedriver for the development of a new generation of low-frequency radio telescope arrays, such as LOFAR, MWA, PAPER, and SKA
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