Cosmological HII bubble growth during reionization

We present general properties of ionized hydrogen (H II) bubbles and their growth based on a state-of-the-art, large-scale (100 Mpc h(-1)) 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 the following: (1) For significant H II bubbles, the number distribution is peaked at a volume of similar to 0.6 Mpc(3) h(-3) at all redshifts. But at z <= 10, one large, connected network of bubbles dominates the entire H II volume. (2) H ii bubbles are highly nonspherical. (3) The H ii regions are highly biased with respect to the underlying matter distribution, with the bias decreasing with time. (4) The non-Gaussianity of the H II region is small when the universe becomes 50% ionized. The non-Gaussianity reaches its maximum near the end of the reionization epoch z similar to 6. But at all redshifts of interest there is a significant non-Gaussianity in the H ii field. (5) Population III galaxies may play a significant role in the reionization process. Small bubbles are initially largely produced by Population III stars. At z >= 10 even the largest H II bubbles have a balanced ionizing photon contribution from Population II and Population III stars, while at z <= 8 Population II stars start to dominate the overall ionizing photon production for large bubbles, although Population III stars continue to make a nonnegligible 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 large bubbles.