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The intergalactic medium is expected to clump on scales down to $10^4-10^8$M$_{\odot}$ before the onset of reionization. The impact of these small-scalestructures on reionization is poorly understood despite the modernunderstanding that gas clumpiness limits the growth of H II regions. We use asuite of radiation-hydrodynamics simulations that capture the $\sim 10^4$$M_\odot$ Jeans mass of unheated gas to study density fluctuations duringreionization. Our simulations track the complex ionization and hydrodynamicalresponse of gas in the wake of ionization fronts. The clumping factor ofionized gas (proportional to the recombination rate) rises to a peak value of$5-20$ approximately $\Delta t = 10$ Myr after ionization front passage,depending on the incident intensity, redshift, and degree to which the gas hadbeen pre-heated by the first X-ray sources. The clumping factor reaches itsrelaxed value of $\approx 3$ by $\Delta t = 300$ Myr. The mean free path ofLyman-limit photons evolves in unison, being up to several times shorter inun-relaxed, recently reionized regions compared to those that were reionizedmuch earlier. Assessing the impact of this response on the global reionizatonprocess, we find that un-relaxed gaseous structures boost the total number ofrecombinations by $\approx 50$ % and lead to spatial fluctuations in the meanfree path that persist appreciably for several hundred million years after thecompletion of reionization.

Hydrodynamic Response of the Intergalactic Medium to Reionization