  Recent VLT SINFONI observations of the close environments (~30pc) of nearby
AGNs have shown that thick gas tori and starbursts with ages between 10 and
150Myr are frequently found. By applying these observations to a previously
established analytical model of clumpy accretion disks, we suggest an
evolutionary sequence for starburst and AGN phases. Whereas the observed
properties of the gas tell us about the current state of the torus, the
starburst characteristics provide information on the history of the torus. In
the suggested evolution, a torus passes through 3 different phases
predetermined by an external mass accretion rate. Started by an initial, short,
and massive gas infall, a turbulent and stellar wind-driven Q~1 disk is formed
in which the starburst proceeds. Once the supernovae explode the intercloud
medium is removed, leaving a massive, geometrically thick, collisional disk
with a decreasing, but still high-mass accretion rate. When the mass accretion
rate has significantly decreased, the collisional torus becomes thin and
transparent as the circumnuclear disk in the Galactic center of the Milky Way.
Variations on this scenario are possible either when there is a second short
and massive gas infall, in which case the torus may switch back into the
starburst mode, or when there is no initial short massive gas infall. All
observed tori up to now have been collisional and thick. The observations show
that this phase can last more than 100Myr. During this phase the decrease in
the mass accretion rate within the torus is slow (a factor of 4 within 150Myr).
The collisional tori also form stars, but with an efficiency of about 10% when
compared to a turbulent disk.
