The interstellar and circumnuclear medium of active nuclei traced by HI
  21-cm absorption
  This review summarises what we have learnt in the last two decades based on
HI 21-cm absorption observations about the cold interstellar medium (ISM) in
the central regions of active galaxies and the interplay between this gas and
the active nucleus (AGN). HI absorption is a powerful tracer on all scales,
from the pc scales to many tens of kpc. Given the strong radio continuum often
associated with the central activity, HI absorption can be used to study the HI
near an AGN out to much higher redshifts than is possible using HI emission. HI
absorption has been used to characterise the general ISM in active galaxies, to
trace the fuelling of radio-loud AGN, to study the feedback between the energy
released by the AGN and the ISM, and the impact of such interactions on the
evolution of galaxies and 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 HI. The structure of these disks has been
traced down to parsec scales by VLBI observations. Young and recently restarted
radio galaxies appear to have a high detection rate of HI. This is interesting
in connection with the evolution of these AGN. This is confirmed by the
discovery of fast, AGN-driven outflows of cold gas which give a direct view of
the impact of the energy released by AGN. 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 HI
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.
