Simulating cosmic reionization at large scales  I. The geometry of reionization
Abstract
We present the first largescale radiative transfer simulations of cosmic reionization, in a simulation volume of (100h^{1} Mpc)^{3}. This is more than a two orders of magnitude improvement over previous simulations. We achieve this by combining the results from extremely large, cosmological, Nbody simulations with a new, fast and efficient code for 3D radiative transfer, C^{2}RAY, which we have recently developed. These simulations allow us to do the first numerical studies of the largescale structure of reionization which at the same time, and crucially, properly take account of the dwarf galaxy ionizing sources which are primarily responsible for reionization. In our realization, reionization starts around z ~ 21, and final overlap occurs by z ~ 11. The resulting electronscattering optical depth is in good agreement with the firstyear Wilkinson Microwave Anisotropy Probe (WMAP) polarization data. We show that reionization clearly proceeded in an insideout fashion, with the highdensity regions being ionized earlier, on average, than the voids. Ionization histories of smallersize (510 comoving Mpc) subregions exabit a large scatter about the mean and do not describe the global reionization history well. This is true even when these subregions are at the mean density of the universe, which shows that smallbox simulations of reionization have little predictive power for the evolution of the mean ionized fraction. The minimum reliable volume size for such predictions is ~30Mpc. We derive the power spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcmin and larger scales. We find two populations of HII regions according to their size, numerous, midsized (~10Mpc) regions and a few, rare, very large regions tens of Mpc in size. Thus, local overlap on fairly large scales of tens of Mpc is reached by z ~ 13, when our volume is only about 50 per cent ionized, and well before the global overlap. We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly nonGaussian. All these quantities are critical for predicting and interpreting the observational signals from reionization from a variety of observations like 21cm emission, Lyα emitter statistics, GunnPeterson optical depth and smallscale cosmic microwave background secondary anisotropies due to patchy reionization.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 July 2006
 DOI:
 10.1111/j.13652966.2006.10502.x
 arXiv:
 arXiv:astroph/0512187
 Bibcode:
 2006MNRAS.369.1625I
 Keywords:

 radiative transfer: methods: numerical: ISM: bubbles: HII regions: galaxies: formation: galaxies: haloes: galaxies: highredshift: intergalactic medium: cosmology: theory;
 radiative transfer;
 methods: numerical;
 ISM: bubbles;
 HII regions;
 galaxies: formation;
 galaxies: haloes;
 galaxies: highredshift;
 intergalactic medium;
 cosmology: theory;
 Astrophysics
 EPrint:
 Comments: 17 pages, 17 figures, replaced to match the published version in MNRAS. Movies and higher resolution figures can be found at http://www.cita.utoronto.ca/~iliev/research.html