Molecular tori in Seyfert galaxies - Feeding the monster and hiding it

Much evidence exists that the majority of Seyfert nuclei are surrounded by a geometrically and optically thick torus of dusty, obscuring gas a few parsecs from the center. We discuss the principal properties of the gas in the torus. It is almost certainly not smoothly distributed; most of its mass is gathered into clouds. The balance between cloud merger and tidal shearing ensures that the covering factor of these clouds in the axial direction is of order unity and determines the shape of the cloud-size distribution function. Orbital energy losses from cloud-cloud collisions drive a net inward flow which matches the previously determined mass efflux in the X-ray-heated wind issuing through the torus's central hole if the average cloud column density is ~10²4^ cm^-2^. Such thick clouds are near the Jeans limit and also ensure that the torus is opaque even to X-rays. Where the gravitational potential is dominated by stars, cloud-cloud collisions keep the molecular clouds close to the equatorial plane. Stirring by stellar processes (e. g. , supernovae or stellar winds) is never strong enough to compete with collisional losses. Close to the nucleus, where the black hole dominates the gravity, there is sufficient orbital shear that viscous "heating" due to the same cloud-cloud collisions may overcome dissipation and produce a cloud velocity dispersion comparable in magnitude to the orbital speed, swelling the torus. The position of the inner edge of the torus is determined by a balance between the inward flow of clouds and the rate at which the nuclear continuum can evaporate them. It is both physically likely, and necessary to the maintenance of that balance, that clouds of particularly low angular momentum (created by the same stirring mechanism which keeps the torus geometrically thick) are captured by the central object, providing its accretion fuel.