This review summarises what we have learnt in the last two decades based on H i 21 cm absorption observations about the cold interstellar medium (ISM) in the central regions of active galaxies and about the interplay between this gas and the active nucleus (AGN). H i absorption is a powerful tracer on all scales, from the parsec-scales close to the central black hole to structures of many tens of kpc tracing interactions and mergers of galaxies. Given the strong radio continuum emission often associated with the central activity, H i absorption observations can be used to study the H i near an active nucleus out to much higher redshifts than is possible using H i emission. In this way, H i absorption has been used to characterise in detail the general ISM in active galaxies, to trace the fuelling of radio-loud AGN, to study the feedback occurring between the energy released by the active nucleus and the ISM, and the impact of such interactions on the evolution of galaxies and of their AGN. In the last two decades, significant progress has been made in all these areas. It is now well established that many radio loud AGN are surrounded by small, regularly rotating gas disks that contain a significant fraction of H i. The structure of these disks has been traced down to parsec scales by very long baseline interferometry observations. Some groups of objects, and in particular young and recently restarted radio galaxies, appear to have a particularly high detection rate of H i. This is interesting in connection with the evolution of these AGN and their impact on the surrounding ISM. This is further confirmed by an important discovery, made thanks to technical upgrades of radio telescopes, namely the presence of fast, AGN-driven outflows of cold gas which give a direct view of the impact of the energy released by AGN on the evolution of galaxies (AGN feedback). In addition, evidence has been collected that clouds of cold gas can play a role in fuelling the nuclear activity. This review ends by briefly describing the upcoming large, blind H i absorption surveys planned for the new radio telescopes which will soon become operational. These surveys will allow to significantly expand existing work, but will also allow to explore new topics, in particular, the evolution of the cold ISM in AGN.

中文翻译：

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H i 21 cm 吸收追踪的活动核的星际和核周介质

这篇综述总结了我们在过去二十年中基于对活动星系中心区域的冷星际介质 (ISM) 以及这种气体与活动核 (AGN) 之间相互作用的 H i 21 厘米吸收观测所学到的知识。Hi 吸收是所有尺度上的强大示踪剂，从靠近中央黑洞的秒差距到数十个 kpc 追踪星系的相互作用和合并的结构。鉴于通常与中心活动相关的强射电连续谱发射，H i 吸收观测可用于研究活动核附近的 H i 比使用 Hi 发射可能发生的红移高得多的红移。通过这种方式，H i 吸收已被用于详细描述活动星系中的一般 ISM，以追踪无线电响 AGN 的燃料，研究活动核释放的能量与 ISM 之间发生的反馈，以及这种相互作用对星系及其 AGN 演化的影响。在过去的二十年里，所有这些领域都取得了重大进展。现在已经确定，许多无线电响亮 AGN 被小的、规则旋转的气体盘包围，这些气体盘包含很大一部分 H i。通过非常长的基线干涉测量观测，这些圆盘的结构可以追溯到秒差距。一些天体，特别是年轻的和最近重新启动的射电星系，似乎对 Hi 的探测率特别高。这与这些 AGN 的演变及其对周围 ISM 的影响有关。一项重要发现进一步证实了这一点，由于射电望远镜的技术升级，即存在快速的、由 AGN 驱动的冷气体流出，可以直接观察 AGN 释放的能量对星系演化的影响（AGN 反馈）。此外，已经收集到的证据表明，冷气体云可以在为核活动提供燃料方面发挥作用。这篇综述最后简要描述了即将投入使用的新射电望远镜计划进行的大型、盲目的 Hi 吸收测量。这些调查将允许显着扩展现有工作，但也将允许探索新主题，特别是 AGN 中冷 ISM 的演变。AGN 驱动的冷气体流出，可以直接观察 AGN 释放的能量对星系演化的影响（AGN 反馈）。此外，已经收集到的证据表明，冷气体云可以在为核活动提供燃料方面发挥作用。这篇综述最后简要描述了即将投入使用的新射电望远镜计划进行的大型、盲目的 Hi 吸收测量。这些调查将允许显着扩展现有工作，但也将允许探索新主题，特别是 AGN 中冷 ISM 的演变。AGN 驱动的冷气体流出，可以直接观察 AGN 释放的能量对星系演化的影响（AGN 反馈）。此外，已经收集到的证据表明，冷气体云可以在为核活动提供燃料方面发挥作用。这篇综述最后简要描述了即将投入使用的新射电望远镜计划进行的大型、盲目的 Hi 吸收测量。这些调查将允许显着扩展现有工作，但也将允许探索新主题，特别是 AGN 中冷 ISM 的演变。计划对即将投入使用的新射电望远镜进行盲测。这些调查将允许显着扩展现有工作，但也将允许探索新主题，特别是 AGN 中冷 ISM 的演变。计划对即将投入使用的新射电望远镜进行盲测。这些调查将允许显着扩展现有工作，但也将允许探索新主题，特别是 AGN 中冷 ISM 的演变。