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arXiv 2403.03243astro-ph.GA
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恒星形成星系中弥散电离气体的性质

The nature of diffuse ionised gas in star-forming galaxies

William McClymont, Sandro Tacchella, Aaron Smith, Rahul Kannan, Roberto Maiolino, Francesco Belfiore, Lars Hernquist, Hui Li, Mark Vogelsberger

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15 pages, 13 figures, submitted to MNRAS, comments welcome
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AI总结：

本研究通过含实时辐射转移的类银河系高分辨率模拟，结合COLT代码分析DIG性质，明确其电子密度阈值与电离源，验证谱线比趋势源于恒星演化导致的辐射场变硬，为恒星相关模型提供检验依据。

摘要：

我们对一个孤立的类银河系星系的高分辨率模拟中的弥散电离气体（DIG）开展了分析，该模拟纳入了实时辐射转移和非平衡热化学过程。我们使用蒙特卡罗辐射转移代码COLT，在后处理过程中自洽地获取电离态和谱线发射信息。我们发现电离气体的电子密度（$n_{\rm e}$）呈现清晰的双峰分布，据此我们定义$n_{\rm e}=10\text{cm}^{-3}$为区分DIG与HII区的阈值。DIG主要由年龄为5-25百万年（Myr）的恒星电离，这些恒星在HII区被清除后直接暴露于低密度气体中。近期形成的恒星（年龄<5 Myr）的辐射泄漏对DIG电离的贡献仅为中等程度。我们对本地观测结果进行正演建模，并将模拟得到的DIG与观测到的[SII]/H$α$、[NII]/H$α$、[OI]/H$α$和[OIII]/H$β$谱线比随$Σ_{\rm Hα}$变化的关系进行对比验证。模拟观测不仅复现了观测到的相关性，还表明这些趋势与温度升高、以及电离辐射场随$n_{\rm e}$降低而变硬的现象相关。DIG内部辐射变硬的原因是，随着$n_{\rm e}$降低，主导电离源从0 Myr恒星逐渐过渡到25 Myr恒星，后者的固有电离光谱逐渐变硬，这主要是由双星相互作用导致的长沃尔夫-拉叶星（Wolf-Rayet）阶段引起的。因此，DIG的谱线比趋势可归因于持续的恒星形成，而非次级电离源，故而可为恒星反馈和星族模型提供有力的检验。

英文摘要：

We present an analysis of the diffuse ionised gas (DIG) in a high-resolution simulation of an isolated Milky Way-like galaxy, incorporating on-the-fly radiative transfer and non-equilibrium thermochemistry. We utilise the Monte-Carlo radiative transfer code COLT to self-consistently obtain ionisation states and line emission in post-processing. We find a clear bimodal distribution in the electron densities of ionised gas ($n_{\rm e}$), allowing us to define a threshold of $n_{\rm e}=10\,\mathrm{cm}^{-3}$ to differentiate DIG from HII regions. The DIG is primarily ionised by stars aged 5-25 Myr, which become exposed directly to low-density gas after HII regions have been cleared. Leakage from recently formed stars ($<5$ Myr) is only moderately important for DIG ionisation. We forward model local observations and validate our simulated DIG against observed line ratios in [SII]/H$α$, [NII]/H$α$, [OI]/H$α$, and [OIII]/H$β$ against $Σ_{\rm Hα}$. The mock observations not only reproduce observed correlations, but also demonstrate that such trends are related to an increasing temperature and hardening ionising radiation field with decreasing $n_{\rm e}$. The hardening of radiation within the DIG is caused by the gradual transition of the dominant ionising source with decreasing $n_{\rm e}$ from 0 Myr to 25 Myr stars, which have progressively harder intrinsic ionising spectra primarily due to the extended Wolf-Rayet phase caused by binary interactions. Consequently, the DIG line ratio trends can be attributed to ongoing star formation, rather than secondary ionisation sources, and therefore present a potent test for stellar feedback and stellar population models.

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