HI absorption in radio galaxies

Twenty-two powerful radio galaxies have been searched for HI absorption. We find the highest probability of detecting HI in absorption among narrow-line compact (or small) galaxies or galaxies with indication of richer interstellar medium (i.e. with ongoing or recent starformation). We discuss the difficulty in the interpretation of the origin of the HI absorption due to the uncertainty in the systemic velocity of the galaxies. A number of interesting features and phenomena are taking place in the central regions of Active Galactic Nuclei (AGN). Among others, and relevant for this work, are: i) the presence of nuclear tori (on the pc-scale) and larger scale disk/tori (extending up to kpc scale and beyond), ii) the possible presence of in-falling gas that has been often suggested as responsible for “feeding” of the central engine, iii) the presence of gas disturbed by the interaction with the radio plasma and finally, iv) the presence of larger-scale tails of gas (that in particular conditions can be seen projected against the nuclear regions) possibly the result of merger or interaction with an other object. The study of all these phenomena requires kinematical information about the gas and therefore one of the few ways to obtain this is by studying the HI absorption against the nuclei of radio galaxies. The advantage of this technique is that observations of the HI absorption against radio continuum sources allow to detect relatively small quantities of HI even in high redshift objects. 1. A well-studied sample Many of the cases of HI absorption detected so far in radio galaxies have been interpreted as evidence for obscuring tori. Thus it can be interesting to investigate