Active galactic nuclei may play a crucial role in regulating the
balance between accretion and ejection of gas in a galaxy.
Absorption of neutral hydrogen (HI) in radio AGN is a powerful tool to identify
cold gas flowing in and out of the active nucleus and to determine the
effects of the interplay between the cold gas and the nuclear activity.
Recently, we observed 250 radio galaxies with the Westerbork Synthesis
Radio Telescope, detecting HI gas in 20% of them. In this talk, I will
show how different shapes, widths and optical depths of the HI
absorption lines that we identified trace different distributions of
gas, such as circumnuclear disks, and infalling and outflowing clouds.
In this talk, I will also present a multi-wavelength study of the cold
gas of PKSB 1718-649. In this young (~100 years) radio-AGN, we detected
two HI absorption lines tracing gas that may fuel the central black
hole. Follow-up SINFONI and ALMA observations allow us to detect
molecular gas that may fuel the nuclear activity. In particular, against
the compact radio emission of the source (r~2pc), we detect carbon
monoxide (CO) gas in absorption at red-shifted velocities with respect
to the systemic velocity (v = +365±22 km/s). This CO (2-1) absorption
line could trace molecular clouds falling onto the central super-massive
black hole. A comparison with the SINFONI observations of the H2 1-0
S(1) line shows that the clouds must be close to the black hole (r<75
pc). The physical conditions of these clouds are different from the gas
at larger radii, and are in good agreement with the predictions for the
conditions of the gas when cold chaotic accretion triggers an active
galactic nucleus. These observations on the centre of PKS B1718-649
provide one of the best indication that a population of cold clouds is
falling towards a radio AGN, likely fuelling its activity
