The impact and response of minihalos and the inter-halo medium on cosmic reionization

An ionization front (I-front) that propagates through an inhomogeneous medium is slowed down by self-shielding and recombinations. We perform cosmological radiation hydrodynamics simulations of the I-front propagation during the epoch of cosmic reionization. The simulations resolve gas in minihalos (halo mass 10^4≲ M_h[ M_⊙]≲ 10^8) that could dominate recombinations, in a computational volume that is large enough to sample the abundance of such halos. The numerical resolution is sufficient (gas particle mass ∼ 20 M_⊙, spatial resolution < 0.1 ckpc) to allow accurate modelling of the hydrodynamic response of gas to photo-heating. We quantify the photo-evaporation time of minihalos as a function of M_h and its dependence on the photo-ionization rate, Γ_-12, and the redshift of reionization, z_i. The recombination rate can be enhanced over that of a uniform medium by a factor ∼ 10-20 early on. The peak value increases with Γ_-12 and decreases with z_i, due to the enhanced contribution from minihalos. The clumping factor, c_r, decreases to a factor of a few at ∼ 100 Myr after the passage of the I-front when the minihalos have been photo-evaporated; this asymptotic value depends only weakly on Γ_-12. Recombinations increase the required number of photons per baryon to reionize the Universe by 20-100 per cent, with the higher value occurring when Γ_-12 is high and z_i is low. We complement the numerical simulations with simple analytical models for the evaporation rate and the inverse Strömgren layer. The study also demonstrates the proficiency and potential of SPHM1RT to address astrophysical problems in high-resolution cosmological simulations.