PKS B1718-649: an HI and H2 perspective on the birth of a compact radio
  source
  We present neutral hydrogen (HI) and warm molecular hydrogen (H2)
observations of the young (10^2 years) radio galaxy PKS B1718-649. We study the
morphology and the kinematics of both gas components, focusing, in particular,
on their properties in relation to the triggering of the radio activity. The
regular kinematics of the large scale HI disk, seen in emission, suggests that
an interaction event occurred too long ago to be responsible for the recent
triggering of the radio activity. In absorption, we detect two absorption lines
along the narrow line of sight of the compact (r<2 pc) radio source. The lines
trace two clouds with opposite radial motions. These may represent a population
of clouds in the very inner regions of the galaxy, which may be involved in
triggering the radio activity. The warm molecular hydrogen (H2 1-0 S(1)
ro-vibrational line) in the innermost kilo-parsec of the galaxy appears to be
distributed in a circum-nuclear disk following the regular kinematics of the HI
and of the stellar component. An exception to this behaviour arises only in the
very centre, where a highly dispersed component is detected. These particular
HI and H2 features suggest that a strong interplay between the radio source and
the surrounding ISM is on-going. The physical properties of the cold gas in the
proximity of the radio source may regulate the accretion recently triggered in
this AGN.
