Characteristic scales during reionization

One of the key observables of the reionization era is the distribution of neutral and ionized gas. Recently, Furlanetto, Zaldarriaga and Hernquist developed a simple analytic model to describe the growth of H  ii regions during this era. Here, we examine some of the fundamental simplifying assumptions behind this model and generalize it in several important ways. The model predicts that the ionized regions attain a well‐defined characteristic size R c that ranges from ∼1 Mpc in the early phases to ≳10 Mpc in the late phases. We show that R c is determined primarily by the bias of the galaxies driving reionization; hence measurements of this scale constrain a fundamental property of the first galaxies. The variance around R c , on the other hand, is determined primarily by the underlying matter power spectrum. We then show that increasing the ionizing efficiency of massive galaxies shifts R c to significantly larger scales and decreases the importance of recombinations. These differences can be observed with forthcoming redshifted 21‐cm surveys (increasing the brightness temperature fluctuations by up to a factor of 2 on large scales) and with measurements of small‐scale anisotropies in the cosmic microwave background. Finally, we show that stochastic fluctuations in the galaxy population only broaden the bubble size distribution significantly if massive galaxies are responsible for most of the ionizing photons. We argue that the key results of this model are robust to many of our uncertainties about the reionization process.