宇宙再电离的 excursion-set 理论中的最后穿越
The Last Crossing in Excursion-Set Theory of Cosmic Reionization
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中文总结 AI 辅助
该研究将光子计数屏障最后穿越作为统计量，发展 excursion-set 理论的最后穿越形式体系，结合CROC模拟揭示电离区域源几何与星系分布的关联，为莱曼α透射尖峰研究提供新视角。
中文摘要 AI 辅助
我引入光子计数屏障的最后穿越作为宇宙再电离分析模型的统计量。由于电离星系是密度的偏置示踪体，且其光子在空间光学深度变化的气体中传播，星系与星系际介质（IGM）的关联需要超越全局电离分数的统计量。Furlanetto等人（2004）的 excursion-set 模型通过光子计数屏障的首次穿越识别某点周围的电离泡尺度。我发展了最后穿越形式体系，定义最后穿越尺度 $R_\text{ℓ}$ 为 enclosed 光子预算仍能电离 enclosed 气体的最小尺度，并解析推导其分布。采用Kaurov（2016）的经验屏障框架应用于CROC模拟，我测量了模拟IGM中的两类穿越。最后穿越形式体系将外部电离区域（其光子计数条件在 $R_\text{ℓ}$ 以下失效）与内部电离区域（其轨迹在分辨率尺度上始终高于屏障）区分开来。已分辨的外部区域主要是低密度的，更大的 $R_\text{ℓ}$ 对应更低的密度和更晚的再电离。在固定局域密度、首次穿越尺度和再电离红移的情况下，这些区域相对于匹配的未分辨区域，在小半径处显示出电离光度和星系数密度的不足，而在中等半径处则显示出两者的过量。它们低密度且近源不足的组合表明，莱曼α（Lyα）透射尖峰可能优先出现在这些区域：低密度降低了光学深度，而更大尺度上的源维持了电离状态。最后穿越图像将电离区域的内部源几何与可检验的、围绕透射选定IGM位置的径向星系分布预测关联起来。
英文摘要
I introduce the last crossing of the photon-counting barrier as a statistic for analytical models of cosmic reionization. Because ionizing galaxies are biased tracers of density and their photons propagate through gas of spatially varying opacity, the galaxy-IGM connection requires statistics beyond the global ionized fraction. The excursion-set model of Furlanetto et al. (2004) identifies the ionized-bubble scale around a point with the first crossing of a photon-counting barrier. I develop the last-crossing formalism and define the last-crossing scale, $R_\ell$, as the smallest scale on which the enclosed photon budget still ionizes the enclosed gas. I derive its distribution analytically. Using the empirical-barrier framework of Kaurov (2016) applied to CROC simulations, I measure both crossings in the simulated IGM. The last-crossing formalism separates externally ionized regions, whose photon-counting condition fails below $R_\ell$, from internally ionized regions whose trajectories remain above the barrier to the resolution scale. Resolved external regions are predominantly underdense, with larger $R_\ell$ selecting lower density and later reionization. At fixed local density, first-crossing scale, and reionization redshift, these regions show deficits in ionizing-luminosity and galaxy-number density at small radius relative to matched unresolved regions, and excesses in both at intermediate radius. Their combination of low gas density and a deficit of nearby sources suggests that Ly$α$ transmission spikes may preferentially arise in these regions: low density reduces the opacity, while sources on larger scales maintain the ionized state. The last-crossing picture connects the internal source geometry of ionized regions to a testable prediction for radial galaxy distributions around transmission-selected IGM locations.
