Where have all the black holes gone?

Abstract

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 of310 3 _ MEdd advection will dominate the energy budget while above that rate all the viscously liberated energy is radiated. The radiation eciency declines steeply below that critical rate. This leads to a clear dichotomy between AGN and normal galaxies which is not so much given by dierences in the mass flow rate as by the radiation eciency. At very low mass accretion rates below 510 5 _ MEdd synchrotron emission and Bremsstrahlung dominate the SED, while above 210 4 _ MEdd 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 ndings for AGN duty cycles and the long-term AGN evolution.