  Current mm-interferometers can provide a complete view of the distribution
and kinematics of molecular gas in the circumnuclear disks of nearby galaxies.
High-resolution CO maps are paramount in order to track down the feeding of
active nuclei and quantitatively address the issue of how and for how long
nuclear activity can be sustained in galaxies. Going beyond CO mapping, the use
of more specific molecular tracers of dense gas can probe the feedback
influence of activity on the chemistry and energy balance/redistribution in the
interstellar medium of nearby galaxies, a prerequisite to interpret how
feedback may operate at higher redshift galaxies. In this context we present
the latest results issued from the NUclei of GAlaxies (NUGA) project, a
high-resolution (0.5''-1'') CO survey of low luminosity AGNs conducted with the
IRAM interferometer. The efficiency of gravity torques as a mechanism to
account for the feeding of low luminosity AGNs (LLAGNs) can be analyzed. We
discuss an evolutionary scenario in which gravity torques and viscosity act in
concert to produce recurrent episodes of activity during the typical lifetime
of any galaxy. We also present the results of an ongoing survey allying the
IRAM 30m telescope with the Plateau de Bure Interferometer (PdBI), devoted to
probe the feedback of activity through the study of the excitation and
chemistry of the dense molecular gas in a sample of nearby AGNs and ULIRGs as
well as in a prototypical high-redshift QSO.

Astrophysics