RECOMBINATION IN SMALL-SCALE STRUCTURES DURING THE COSMIC REIONIZATION
* Abstract
Density inhomogeneity in the intergalactic medium (IGM) on sub-Mpc scales can boost the recombination rate of ionized gas substantially, affecting the growth of H II regions during the Epoch of Reionization (EoR). Previous attempts to express this in terms of a clumping factor, C, typically failed to resolve the full range of mass scales which are important in establishing this effect, down to the Jeans scale in the pre-ionization IGM, along with the hydrodynamical back-reaction of reionization. Towards that end, we introduce GADGET-RT, a GADGET code with a new algorithm to transfer H-ionizing radiation, and perform a set of radiation-hydrodynamics simulations from cosmological initial conditions. We extend the mass resolution of previous work to the scale of minihalos (with masses below 108 Mʘ) simulating sub-Mpc volumes. Pre-reionization structure is evolved until a redshift zi at which the ionizing radiation from external sources sweeps supersonically across the volume in a few Myr, until it is trapped on the surfaces of minihalos. Small-scale density structures during this time lead to a high (C >10) clumping factor for ionized gas. This high clumping factor hugely boosts the recombination rate until the structures are mostly disrupted by the hydrodynamic feedback after ~ 10 – 100 Myr. For incoming radiation with intensity J21, a number of extra recombinations result per H atom, on top of what is expected from gas at the mean density, is given by 0.32 [J21]0.12 [(1 + zi)/11]-1.7. In models in which most of the volume is ionized toward the end of reionization, this can add up to ~ 0.7 per H atom to the ionizing photon budget to achieve reionization.
