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 A V 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


氢原子 （Hi） 是银河系星际介质 （ISM） 气体演化周期中的关键垫脚石。Hi 追踪了恒星形成前的寒冷分子前状态和恒星形成前后温暖的弥散 ISM。这篇综述描述了新的、灵敏的 Hi 吸收和发射巡天，这些巡天与高角度和光谱分辨率的 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，以及其结构可以告诉我们有关 ISM 中反馈的信息。主要发现包括以下内容： ▪冷中性介质的质量分数平均为 ≲40%，由于平均气体密度的增加，随 A V 的增加而增加。▪冷盘延伸到至少 R ∼ 25 kpc。▪大约 40% 的 Hi 是暖的，其结构特征来自反馈事件。▪冷 Hi 是高度丝状的， 而暖 Hi 分布更平滑。我们总结了未来的观测和模拟机会，这些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。