The AGN dusty torus -- observational clues for its structure and dynamics

Moshe Elitzur (University of Kentucky)

Evidence for the existence of a dusty toroidal structure around the AGN central engine comes from both obscuration (x-rays and optical) and emission (x-rays and infrared). The only measured torus properties are its absorbing column of $10^{22}$--$10^{25}$ cm$^{-2}$ (from x-rays) and inner radius of $\sim$ 1--2 pc (from recent IR imaging and interferometry). There are only upper limits on the torus outer radius, of $\sim$ 30--35 pc in both NGC 1068 and NGC 4151. But in spite of these small upper bounds, the general perception is that the torus extends to hundreds of pc. The origin of this belief can be traced to the need of smooth-density models to place dust at large distances to produce sufficient amounts of cool dust emission. The recent incorporation of clumpiness in torus modeling alleviates the need for extended torus sizes. The reason is that AGN-illuminated clouds have a dark face, providing cool dust emission even for nearby clouds; even at 4pc away from an AGN with $10^{12} L_\odot$, a cloud can contain dust as cool as ~ 200--300K on its dark side. The case for compact torus sizes is enhanced by recent findings of Lutz et al (04) that the torus IR emission is isotropic, a property reproduced by clumpy torus models with steep radial density profiles. Compact torus sizes give new impetus to a disk-wind origin for the torus, an idea first proposed by Emmering, Blandford \& Shlosman (92) and since advocated by Konigl. Instead of a puffed-up distribution of clouds whose hydrostatic support was never identified, in this paradigm the torus is simply the optically thick region of the wind that uplifts clouds off the central accretion disk.

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