Hydrodynamic Response of the Intergalactic Medium to Reionization

The intergalactic medium is expected to clump on scales down to ${10}^{4}\mbox{--}{10}^{8}$ M_⊙ before the onset of reionization. The impact of these small-scale structures on reionization is poorly understood despite the modern understanding that gas clumpiness limits the growth of ${\rm{H}}\,{\rm{II}}$ regions. We use a suite of radiation-hydrodynamics simulations that capture the $\sim {10}^{4}{M}_{\odot }$ Jeans mass of unheated gas to study density fluctuations during reionization. Our simulations track the complex ionization and hydrodynamical response of gas in the wake of ionization fronts. The clumping factor of ionized gas (proportional to the recombination rate) rises to a peak value of 5-20 approximately Δt = 10 Myr after ionization front passage, depending on the incident intensity, redshift, and degree to which the gas had been preheated by the first X-ray sources. The clumping factor reaches its relaxed value of ≈3 by Δt = 300 Myr. The mean free path of Lyman-limit photons evolves in unison, being up to several times shorter in unrelaxed, recently reionized regions compared to those that were reionized much earlier. Assessing the impact of this response on the global reionization process, we find that unrelaxed gaseous structures boost the total number of recombinations by ≈50% and lead to spatial fluctuations in the mean free path that persist appreciably for several hundred million years after the completion of reionization.