Keeping the Universe ionized: photoheating and the clumping factor of the high-redshift intergalactic medium
Keeping the Universe ionized: photoheating and the clumping factor of the high-redshift intergalactic medium
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DOI：
10.1111/j.1365-2966.2009.14486.x
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发表时间：
2008-07
影响因子：
4.8
通讯作者：
A. Pawlik;J. Schaye;E. Scherpenzeel
中科院分区：
文献类型：
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作者：
A. Pawlik;J. Schaye;E. Scherpenzeel
维持星系间氢离子化所需的临界星星形成率（SFR）密度主要取决于星系间介质（IGM）中的平均再组合率。这个速率与聚集因子C ρ 2 B ρ 2成正比，其中ρB和ρ分别为局域和宇宙平均重子密度，括号中的“IGM”表示IGM中重组气体的空间平均值。我们进行了一套宇宙学光滑粒子流体动力学模拟，包括辐射冷却，以计算在红移z ≥ 6的IGM的体积加权聚集因子。我们专注于光致电离加热均匀的紫外线背景的影响，并发现，光加热强烈地降低了聚团因子，因为增加的压力支持平滑了小尺度的密度波动。光致电离加热通常被认为对IGM的再电离提供负反馈，因为它通过将气体从低质量晕中沸腾出来来抑制宇宙SFR。然而，由于聚集因子的降低，它也使得更容易保持IGM电离。因此，光加热也提供了积极的反馈，虽然已知这种反馈存在，但受到的关注要少得多。我们证明，这种积极的反馈实际上是非常强大的。使用保守的假设，我们发现，如果IGM被重新加热到z 9，恒星形成星系的观察人口在z 10.6可能足以保持IGM电离，提供的电离光子逃离恒星形成区域的分数大于10.2。
The critical star formation rate (SFR) density required to keep the intergalactic hydro- gen ionized depends crucially on the average rate of recombinations in the intergalac- tic medium (IGM). This rate is proportional to the clumping factor C ≡� ρ 2� IGM/�ρ b� 2 , where ρb and �ρ bare the local and cosmic mean baryon density, respectively, and the brackets �� IGM indicate spatial averaging over the recombining gas in the IGM. We per- form a suite of cosmological smoothed particle hydrodynamic simulations that include ra- diative cooling to calculate the volume-weighted clumping factor of the IGM at redshifts z ≥ 6. We focus on the effect of photoionization heating by a uniform ultraviolet background and find that photoheating strongly reduces the clumping factor because the increased pressure support smoothes out small-scale density fluctuations. Photoionization heating is often said to provide a negative feedback on the re-ionization of the IGM because it suppresses the cosmic SFR by boiling the gas out of low-mass haloes. However, because of the reduction of the clumping factor it also makes it easier to keep the IGM ionized. Photoheating therefore also provides a positive feedback which, while known to exist, has received much less attention. We demonstrate that this positive feedback is in fact very strong. Using conservative assumptions, we find that if the IGM was reheated at z 9, the observed population of star-forming galaxies at z ≈ 6 may be sufficient to keep the IGM ionized, provided that the fraction of ionizing photons that escape the star-forming regions to ionize the IGM is larger than ∼0.2.