The impact and response of mini-haloes and the interhalo medium on cosmic reionization

An ionization front (I-front) that propagates through an inhomogeneous medium is slo wed do wn 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 mini-haloes (halo mass 10 4 < Mh[M] < 10 8 ) that could dominate recombinations, in a computational volume that is large enough to sample the abundance of such haloes. The numerical resolution is sufficient (gas-particle mass similar to 20 M and spatial resolution < 0.1 ckpc) to allow accurate modelling of the hydrodynamic response of gas to photoheating. We quantify the photoe v aporation time of mini-haloes as a function of Mh and its dependence on the photoionization rate, F-12, and the redshift of reionization, zi. The recombination rate can be enhanced o v er that of a uniform medium by a factor similar to 10 -20 early on. The peak value increases with F-12 and decreases with zi, due to the enhanced contribution from mini-haloes. The clumping factor, cr, decreases to a factor of a few at similar to 100 Myr after the passage of the I-front when the mini-haloes have been photoe v aporated; this asymptotic v alue depends only weakly on F-12. Recombinations increase the required number of photons per baryon to reionize the Universe by 20 per cent-100 per cent, with the higher value occurring when F-12 is high and zi is low. We complement the numerical simulations with simple analytical models for the e v aporation rate and the inverse Str omgren layer. The study also demonstrates the proficiency and potential of SPH-M1RT to address astrophysical problems in high-resolution cosmological simulations.