We have calculated stationary models for accretion disks around super-massive black holes in galactic nuclei .
Our models show that below a critical mass flow rate of \sim 3 10 ^ { -3 } \dot { M } _ { \mathrm { Edd } } advection will dominate the energy budget while above that rate all the viscously liberated energy is radiated .
The radiation efficiency declines steeply below that critical rate .
This leads to a clear dichotomy between AGN and normal galaxies which is not so much given by differences in the mass flow rate but by the radiation efficiency .
At very low mass accretion rates below 5 10 ^ { -5 } \dot { M } _ { \mathrm { Edd } } synchrotron emission and Bremsstrahlung dominate the SED , while above 2 10 ^ { -4 } \dot { M } _ { \mathrm { Edd } } the inverse Compton radiation from synchrotron seed photons produce flat to inverted SEDs from the radio to X-rays .
Finally we discuss the implications of these findings for AGN duty cycles and the long-term AGN evolution .