Physics & Astronomy Colloquium

3:30 PM, Friday, September 14, 2007
Room 155, Chem-Phys Building

Dr. Moshe Elitzur
Department of Physics and Astronomy
University of Kentucky

"Active Galactic Nuclei: A Paradigm Shift''

The variety of observations of Active Galactic Nuclei (AGN) have been classified with a simple unified scheme: The nuclear activity is powered by a central massive black hole that drives radio emitting jets and ionizes surrounding line-emitting clouds. The whole system is surrounded by a dusty torus, and the observer's orientation with respect to this obscuring torus determines the appearance of the AGN. Pole-on viewing gives rise to superluminal jets and Seyfert 1 line spectrum, edge-on results in extended radio lobes and Seyfert 2 lines. The torus is comprised of optically thick dusty clouds in a rotating configuration with roughly equal vertical and radial dimensions.

Although the observed IR is in broad agreement with the expected effects of the dusty torus, detailed properties of the spectral energy distribution (SED) posed difficult problems. The dynamical origin of the rotating cloud configuration, and especially its vertical support, present an even more serious challenge. We have recently developed a formalism to handle radiative transfer in clumpy media and in this talk I show that the SED problems find a natural explanation if the torus is made of dusty clouds, with about 5--10 clouds along each radial equatorial ray. Furthermore, the properties of the model SED may also provide the answer for the torus dynamical origin and solidify the case for a paradigm shift: the torus is apparently just the dusty region of wind outflow from the AGN accretion disk in which the clouds are optically thick.

Refreshments will be served in CP 179 at 3:15 PM