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.

这篇综述总结了我们在过去二十年中基于 H  i 21 cm 吸收观测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) 之间相互作用的了解。 Hi 吸收是所有尺度上的强大示踪剂，从接近中心黑洞的秒差距尺度到数十 kpc 的结构，追踪星系的相互作用和合并。鉴于强射电连续谱发射通常与中心活动相关，H  i吸收观测可用于研究活动核附近的 H  i ，其红移比使用 H i发射可能高得多 。通过这种方式，H  i吸收已被用来详细表征活动星系中的一般ISM，追踪射电强活动星系核的燃料供给，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活动星系核演化的影响。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，许多无线电响亮的活动星系核被小的、有规律旋转的气盘包围，其中含有很大一部分的 H  i。通过非常长的基线干涉测量观测，这些盘的结构可以追溯到秒差距尺度。一些天体群，特别是年轻的和最近重新启动的射电星系，似乎具有特别高的 H  i检测率。这与这些活动星系核的演化及其对周围 ISM 的影响有关。射电望远镜技术升级带来的一项重要发现进一步证实了这一点，即存在由活动星系核驱动的冷气体快速流出，可以直接观察活动星系核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。这篇评论最后简要描述了即将投入运行的新型射电望远镜计划进行的大型盲 H  i吸收调查。这些调查将显着扩展现有工作，但也将允许探索新主题，特别是活动星系核中冷ISM的演变。