Astronomy & Astrophysics

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Issue		A&A Volume 387, Number 2, May IV 2002
Page(s)		422 - 428
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20020438

A&A 387, 422-428 (2002)
DOI: 10.1051/0004-6361:20020438

Where have all the black holes gone?

T. Beckert¹ and W. J. Duschl^{2, 1}

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Institut für Theoretische Astrophysik, Tiergartenstraße 15, 69121 Heidelberg, Germany

(Received 24 April 2001 / Accepted 18 March 2002)

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 of ${\sim}3\times10^\,\dot{M}_{\rm Edd}$ 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 as by the radiation efficiency. At very low mass accretion rates below $5\times10^\, \dot{M}_{\rm Edd}$ synchrotron emission and Bremsstrahlung dominate the SED, while above $2\times10^\, \dot{M}_{\rm Edd}$ 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.

Key words: accretion, accretion disks -- black hole physics -- radiation mechanisms: non-thermal -- galaxies: active -- galaxies: nuclei

Offprint request: T. Beckert, tbeckert@mpifr-bonn.mge.de

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