The radiation from stars and active galactic nuclei (AGNs) creates photodissociation regions (PDRs) and X-ray-dominated regions (XDRs), where the chemistry or heating is dominated by far-ultraviolet (FUV) radiation or X-ray radiation, respectively. PDRs include a wide range of environments, from the diffuse interstellar medium (ISM) to dense star-forming regions. XDRs are found in the center of galaxies hosting AGNs, in protostellar disks, and in the vicinity of X-ray binaries. In this review, we describe the dominant thermal, chemical, and radiation transfer processes in PDRs and XDRs, as well as give a brief description of models and their use for analyzing observations. We then present recent results from Milky Way, nearby extragalactic, and high-redshift observations.Several important results include the following:▪ Velocity-resolved PDR lines reveal the kinematics of the neutral atomic gas and provide constraints on the stellar feedback process. Their interpretation is, however, in dispute, as observations suggest a prominent role for stellar winds, whereas they are much less important in theoretical models. ▪ A significant fraction of molecular mass resides in CO-dark gas especially in low-metallicity and/or highly irradiated environments.▪ The CO ladder and [Ci]/[Cii] ratios can determine if FUV or X rays dominate the ISM heating of extragalactic sources. ▪ With Atacama Large Millimeter/submillimeter Array, PDR and XDR tracers are now routinely detected on galactic scales over cosmic time. This makes it possible to link the star-formation history of the Universe to the evolution of the physical and chemical properties of the gas.

中文翻译：

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光解离和 X 射线主导区域


来自恒星和活动星系核 (AGN) 的辐射产生光解离区域 (PDR) 和 X 射线主导区域 (XDR)，其中化学或加热分别由远紫外 (FUV) 辐射或 X 射线辐射主导。 PDR 包括广泛的环境，从弥漫的星际介质 (ISM) 到密集的恒星形成区域。 XDR 存在于活动星系核所在的星系中心、原恒星盘以及 X 射线双星附近。在这篇综述中，我们描述了 PDR 和 XDR 中主要的热、化学和辐射传输过程，并简要描述了模型及其用于分析观测结果的用途。然后，我们展示了银河系、附近河外星系和高红移观测的最新结果。几个重要结果包括：▪ 速度分辨的 PDR 线揭示了中性原子气体的运动学，并为恒星反馈过程提供了约束。然而，他们的解释存在争议，因为观测表明恒星风起着突出的作用，而它们在理论模型中的重要性要低得多。 ▪ 分子质量的很大一部分存在于 CO 暗气体中，特别是在低金属丰度和/或高辐射环境中。▪ CO 阶梯和 [Ci]/[Cii] 比率可以确定 FUV 或 X 射线是否主导 ISM 加热河外来源。 ▪ 借助阿塔卡马大型毫米/亚毫米阵列，现在可以在宇宙时间内在银河尺度上常规检测到 PDR 和 XDR 示踪剂。这使得将宇宙的恒星形成历史与气体物理和化学性质的演化联系起来成为可能。