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-scale structures on reionization is poorly understood despite the modern understanding that gas clumpiness limits the growth of H 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 $\Delta t = 10$ Myr after ionization front passage, depending on the incident intensity, redshift, and degree to which the gas had been pre-heated by the first X-ray sources. The clumping factor reaches its relaxed value of $\approx 3$ by $\Delta t = 300$ Myr. The mean free path of Lyman-limit photons evolves in unison, being up to several times shorter in un-relaxed, recently reionized regions compared to those that were reionized much earlier. Assessing the impact of this response on the global reionizaton process, we find that un-relaxed gaseous structures boost the total number of recombinations by $\approx 50$ % and lead to spatial fluctuations in the mean free path that persist appreciably for several hundred million years after the completion of reionization.

中文翻译：

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星际介质对再电离的流体动力学响应

在再电离开始之前，星系间介质预计会聚集到 $10^4-10^8$ M$_{\odot}$。尽管现代认为气体块状限制了 H II 区域的生长，但人们对这些小尺度结构对再电离的影响知之甚少。我们使用一套辐射流体动力学模拟来捕获未加热气体的 $\sim 10^4$ M$_{\odot}$ Jeans 质量，以研究再电离过程中的密度波动。我们的模拟跟踪了电离前沿后气体的复杂电离和流体动力学响应。电离气体的聚集因子（与复合率成正比）上升到 $5-20$ 的峰值，大约 $\Delta t = 10$ Myr 在电离前沿通过后，取决于入射强度、红移、气体被第一个 X 射线源预热的程度。结块因子达到其松弛值 $\approx 3$ 乘以 $\Delta t = 300$ Myr。莱曼极限光子的平均自由程是一致演化的，在未松弛的、最近重新电离的区域中，与那些更早重新电离的区域相比，其平均自由程要短几倍。评估这种响应对全球再电离过程的影响，我们发现未松弛的气态结构使复合总数增加了约 50% %，并导致平均自由程的空间波动，这种波动明显持续了数亿再电离完成后数年。与那些更早被重新电离的区域相比，在未放松的、最近重新电离的区域中的时间要短几倍。评估这种响应对全球再电离过程的影响，我们发现未松弛的气态结构使复合总数增加了约 50% %，并导致平均自由程的空间波动，这种波动明显持续了数亿再电离完成后数年。与较早重新电离的区域相比，在未放松的、最近重新电离的区域中的时间要短几倍。评估这种响应对全球再电离过程的影响，我们发现未松弛的气态结构使复合总数增加了约 50% %，并导致平均自由程的空间波动，这种波动明显持续了数亿再电离完成后数年。