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arXiv 2311.06348astro-ph.COastro-ph.GA
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再电离对平均自由程直接测量的影响

The effect of reionization on direct measurements of the mean free path

Joshua T. Roth, Anson D'Aloisio, Christopher Cain, Bayu Wilson, Yongda Zhu, George D. Becker

备注：
Accepted to MNRAS, minor changes
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AI总结：

本研究利用辐射转移模拟探讨再电离中性岛对类星体复合光谱测量电离光子平均自由程的影响，发现中性岛对透射的抑制与光谱归一化简并，不会显著偏倚推断结果。

摘要：

近期基于复合类星体光谱的电离光子平均自由程（MFP）测量可能表明再电离在 $z<6$ 结束。这些测量具有挑战性，因为它们依赖于对类星体近邻区域的假设。例如，某些类星体可能靠近仍在进行再电离的中性区域（“中性岛”），目前尚不清楚它们如何影响测量。我们通过辐射转移模拟中的模拟 MFP 测量来探讨此问题。我们发现，即使存在中性岛，我们的模拟 MFP 测量结果与模拟中真实的平均空间 MFP（包含电离介质和岛屿的不透明度）一致性在 $30~\\%$ 以内。对于现实模型，推断的 MFP 对类星体环境和寿命假设的敏感度低于 $50\\%$。我们证明，未来使用改进数据的分析可能需要显式建模中性岛对复合光谱的影响，并概述了实现方法。最后，我们量化了中性岛对 Lyman 系列透射的影响，此效应在先前的 MFP 分析中曾用光学薄模拟建模。中性岛会显著抑制 $\lambda_{\rm rest}<912$ 埃的透射，在 $z_{\rm qso}=6$ 的合理再电离场景下抑制可达 2 倍，原因是类星体光红移进入共振时被许多密集谱线吸收。然而，该抑制与光谱归一化几乎完全简并，因此不会显著偏倚推断的 MFP。

英文摘要：

Recent measurements of the ionizing photon mean free path (MFP) based on composite quasar spectra may point to reionization ending at $z<6$. These measurements are challenging because they rely on assumptions about the proximity zones of the quasars. For example, some quasars might have been close to neutral patches where reionization was still ongoing ("neutral islands"), and it is unclear how they would affect the measurements. We address this question with mock MFP measurements from radiative transfer simulations. We find that, even in the presence of neutral islands, our mock MFP measurements agree to within $30~\%$ with the true spatially averaged MFP in our simulations, which includes opacity from both the ionized medium and the islands. The inferred MFP is sensitive at the $<~50\%$ level to assumptions about quasar environments and lifetimes for realistic models. We demonstrate that future analyses with improved data may require explicitly modeling the effects of neutral islands on the composite spectra, and we outline a method for doing this. Lastly, we quantify the effects of neutral islands on Lyman-series transmission, which has been modeled with optically thin simulations in previous MFP analyses. Neutral islands can suppress transmission at $λ_{\rm rest}<912$ Angstroms significantly, up to a factor of 2 for $z_{\rm qso}=6$ in a plausible reionization scenario, owing to absorption by many closely spaced lines as quasar light redshifts into resonance. However, the suppression is almost entirely degenerate with the spectrum normalization and thus does not significantly bias the inferred MFP.

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