Abstract
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.
Highlights
As is well known, hydrogen is the most common element in the Universe and it is observed in structures which span from the largest cosmological scales to the small, pcscale of the centres of galaxies
In this review we have summarised the main results based on H i 21 cm absorption observations obtained in the last two decades about the cold interstellar medium (ISM) in the central regions of radio loud galaxies and about the interplay between this gas and the AGN
Given the specific nature of H i absorption observations, they make it possible to study the cold gas in the environment of AGN at parsec scales at higher redshift and can probe a completely different parameter space than observations of H i emission
Summary
Hydrogen is the most common element in the Universe and it is observed in structures which span from the largest cosmological scales to the small, pcscale of the centres of galaxies. The statistics of the shapes and widths of H i absorption lines gives information on whether the ISM is likely to be in a regularly rotating structure or whether there are indications for more irregular gas distributions and indirectly on the role of galaxy interactions Such studies give information on to what extent different kinds of large- and small-scale gas structures are found in different types of AGN which, in turn, tells something about the evolution of the different types of objects. Which has seen major developments in recent years, is the detection of fast AGN-driven outflows of cold gas, detected as broad, blue-shifted wings in the H i absorption profiles Such outflows are a manifestation of AGN feedback which is an important ingredient in current models for galaxy evolution and of the evolution of their central super massive black hole. For emission and for absorption studies one uses exactly the same type of observations
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