Cosmological Hii Bubble Growth during Reionization

We present general properties of ionized hydrogen (HII) bubbles and their growth based on a stateof-the-art large-scale (100 Mpc/h) cosmological radiative transfer simulation. The simulation resolves all halos with atomic cooling at the relevant redshifts and simultaneously performs radiative transfer and dynamical evolution of structure formation. Our major conclusions include: (1) for significant HII bubbles, the number distribution is peaked at a volume of ∼ 0.6Mpc/h at all redshifts. But, at z ≤ 10, one large, connected network of bubbles dominates the entire HII volume. (2) HII bubbles are highly non-spherical. (3) The HII regions are highly biases with respect to the underlying matter distribution with the bias decreasing with time. (4) The universe becomes 50% ionized at redshfit z ∼ 9.5. The non-gaussianity of the HII region reaches its maximal near the end of the reionization epoch z ∼ 6. But at all redshifts of interest there are significant non-gaussianity in the HII field. (5) Population III galaxies play a very important role in the reionization process. Small bubbles are initially largely produced by Pop III stars. At z ≥ 10 even the largest HII bubbles have a balanced ionizing photon contribution from Pop II and Pop III stars, while at z ≤ 8 Pop II stars start to dominate the overall ionizing photon production for large bubbles, although Pop III stars continue to make a non-negligible contribution. (6) The relationship between halo number density and bubble size is complicated but a strong correlation is found between halo number density and bubble size for for large bubbles. Subject headings: cosmology: theory—large-scale structure of universe—early universe —galaxies: intergalactic medium—methods: numerical—radiative transfer