森林暗处：利用IllustrisTNG对高柱密度系统进行以暗晕为中心的表征
Where the Forest Goes Dark: Halo-centered Characterization of High Column Density Systems with IllustrisTNG
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- University of São Paulo（圣保罗大学）
- The Barcelona Institute of Science and Technology（巴塞罗那科学技术研究所）
- Columbia University（哥伦比亚大学）
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中文总结 AI 辅助
本研究利用TNG50模拟，以暗晕为中心表征高柱密度系统，发现吸收类型随碰撞参数转变且与质量无关，为前向模拟莱曼α森林污染奠定基础。
中文摘要 AI 辅助
中性氢的高柱密度系统（HCDs）——莱曼极限系统（LLSs）、亚阻尼及阻尼莱曼α吸收体（DLAs）——会污染莱曼α森林的3D和1D统计量。大多数DLA在分析中被剔除，但清洗算法并不完美，且较弱的LLS无法单独检测，因此残余污染被吸收进噪声参数中，削弱了宇宙学约束能力。模拟为研究这一效应提供了理想环境，使得吸收特征可以直接追溯到产生它们的气体。我们在TNG50流体动力学模拟的气体单元上识别高柱密度结构，在速度空间中对它们进行去混合，并将每条视线分割为仅含HCD和仅含森林的光谱。应用于红移z≈3、半径达50倍维里半径的暗晕，该方法得到了每类结构随碰撞参数b和暗晕质量的覆盖比例。碰撞参数决定了光谱中出现的吸收类型：在b≈0.11 R_200c处，DLA让位于亚DLA；在0.3处，亚DLA让位于LLS；在0.5处，LLS让位于莱曼α森林，后者在R_200c处覆盖了约78%的视线。以R_200c为单位，该序列在三个数量级的质量范围内几乎与质量无关，因此维里半径设定了中性气体分布的尺度。单个沃伊特分量可以恢复几乎所有阻尼系统的柱密度，但无法恢复LLS的柱密度，因为LLS的吸收特征通常来自视线方向上多个气体结构的单独贡献。我们对HCDs的详细表征为构建直接前向模拟其对莱曼α光谱贡献的先进技术迈出了第一步。
英文摘要
High column density systems of neutral hydrogen (HCDs) --- Lyman limit systems (LLSs), sub-damped and damped Ly$α$ absorbers (DLAs) --- contaminate both the 3D and the 1D statistics of the Ly$α$ forest. Most DLAs are masked out from the analyses, but cleaning algorithms are not perfect, and weaker LLSs are individually undetectable, so the residual contamination is absorbed into nuisance parameters that dilute the cosmological constraining power. Simulations provide the ideal setting to study this effect, allowing absorption features to be directly traced back to the gas producing them. We identify high-column-density structures on the gas cells of the TNG50 hydrodynamical simulation, deblend them in velocity space, and split every sightline into HCD-only and forest-only spectra. Applied to halos at $z \simeq 3$ out to 50 virial radii, it yields each class's covering fraction against impact parameter $b$ and halo mass. The impact parameter defines the type of absorption appearing in the spectrum: DLAs give way to sub-DLAs at $b \approx 0.11\,R_{200c}$, sub-DLAs to LLSs at $0.3$, and LLSs to the Ly$α$-forest at $0.5$, which covers $\simeq 78\%$ of sightlines at $R_{200c}$. In units of $R_{200c}$ the sequence is nearly mass-independent over three decades in mass, so the virial radius sets the scale of the neutral gas distribution. A single Voigt component recovers the column density of essentially every damped system, but not of LLSs, whose absorption features often arise from several separate contributions from gas structures along the sightline. Our detailed characterization of HCDs gives the first step to the construction of advanced techniques to directly forward-model their contribution to Ly$α$ spectra.
