Atomic hydrogen (Hi) is a critical stepping stone in the gas evolution cycle of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold, premolecular state before star formation and the warm, diffuse ISM before and after star formation. This review describes new, sensitive Hi absorption and emission surveys, which, together with high angular and spectral resolution Hi emission data, have revealed the physical properties of Hi, its structure, and its association with magnetic fields. We give an overview of the Hi phases and discuss how Hi properties depend on the environment and what its structure can tell us about feedback in the ISM. Key findings include the following: ▪ The mass fraction of the cold neutral medium is ≲40% on average, increasing with AV due to the increase of mean gas density. ▪ The cold disk extends to at least R ∼ 25 kpc. ▪ Approximately 40% of the Hi is warm, with structural characteristics that derive from feedback events. ▪ Cold Hi is highly filamentary, whereas warm Hi is more smoothly distributed.We summarize future observational and simulation opportunities that can be used to unravel the 3D structure of the atomic ISM and the effects of heating and cooling on Hi properties.

中文翻译：

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银河系中的原子氢：星系演化的垫脚石

原子氢（Hi）是银河系星际介质（ISM）气体演化循环中的关键垫脚石。 Hi 追踪了恒星形成之前的冷的分子前状态以及恒星形成之前和之后温暖的、弥漫的ISM。这篇综述描述了新的、灵敏的 Hi 吸收和发射调查，这些调查与高角度和光谱分辨率 Hi 发射数据一起，揭示了 Hi 的物理特性、其结构及其与磁场的关联。我们概述了 Hi 阶段，并讨论了 Hi 属性如何依赖于环境，以及它的结构可以告诉我们 ISM 中的反馈信息。主要发现如下： ▪ 冷中性介质的质量分数平均为 ≲40%，随着 A 的增加而增加V由于平均气体密度的增加。 ▪ 冷盘延伸至至少R ∼ 25 kpc。 ▪ 大约 40% 的 Hi 是温暖的，具有源自反馈事件的结构特征。 ▪ 冷Hi 高度丝状，而暖Hi 分布更平滑。我们总结了未来的观测和模拟机会，可用于揭示原子ISM 的3D 结构以及加热和冷却对Hi 特性的影响。