The morphology of H II regions during reionization

Abstract

It is possible that the properties of H II regions during reionization depend sensitively on many poorly constrained quantities [the nature of the ionizing sources, the clumpiness of the gas in the intergalactic medium (IGM), the degree to which photoionizing feedback suppresses the abundance of low-mass galaxies, etc.], making it extremely difficult to interpret upcoming observations of this epoch. We demonstrate that the actual situation is more encouraging, using a suite of radiative transfer simulations, post-processed on outputs from a 10243, 94-Mpc N-body simulation. Analytic prescriptions are used to incorporate small-scale structures that affect reionization, yet remain unresolved in the N-body simulation. We show that the morphology of the H II regions for reionization by POPII-like stars is most dependent on the global ionization fraction (x) over bar (i). Changing other parameters by an order of magnitude for fixed (x) over bar (i) often results in similar bubble sizes and shapes. The next most important dependence is on the properties of the ionizing sources. The rarer the sources, the larger and more spherical the H II regions become. The typical bubble size can vary by as much as a factor of 4 at fixed (x) over bar (i) xi between different possible source prescriptions. The final relevant factor is the abundance of minihaloes or of Lyman-limit systems. These systems suppress the largest bubbles from growing, and the magnitude of this suppression depends on the thermal history of the gas as well as the rate at which these systems are photo-evaporated. We find that neither source suppression owing to photo-heating nor small-scale gas clumping significantly affects the large-scale structure of the H II regions, with the ionization fraction power spectrum at fixed (x) over bar (i) differing by less than 20 per cent for k < 5 Mpc(-1) between all the source suppression and clumping models we consider. Analytic models of reionization are successful at predicting many of the features seen in our simulations. We discuss how observations of the 21-cm line with the Mileura Widefield Array (MWA) and the Low Frequency Array (LOFAR) can constrain properties of reionization, and we study the effect patchy reionization has on the statistics of Ly alpha emitting galaxies.