This thesis aims to make an inventory of the occurrence of neutral hydrogen (HI) traced by absorption in nearby radio galaxies, derive the properties of the gas and study the interplay between the atomic and molecular gas and the active nucleus (AGN). We focus on two projects to link the gas in the central regions of galaxies to the triggering, fuelling and feedback effects of the radio AGN.

The first project investigates the presence of HI absorption in a sample of radio AGN to link the properties of the lines to the the evolutionary stage of the radio activity as well as to the properties of the dust, that we infer from the mid-infrared colours of the sources. We present the first comprehensive statistics of HI outflows, albeit derived from shallow observations. We also consider the sources where HI absorption is not detected to perform stacking experiments that allow us to probe the general properties of HI in radio AGN.

The second project is a multi-wavelength study of a young radio AGN (PKS B17181-649) to understand how cold gas may trigger the radio nuclear activity. We observed the HI, the molecular hydrogen and the carbon monoxide gas. All these components indicate that cold gas is contributing to fuel the central nuclear activity. We found evidence for molecular clouds accreting onto the central supermassive black hole.

To conclude we discuss on the implications of the results of this thesis results on future research projects, with upcoming HI surveys from the SKA pathfinders/precursors.