Physical Conditions in Obscuring Tori and Molecular Accretion Disks, and   Are They Really the Same Thing?
  The nature of the obscuring material in active galactic nuclei is still uncertain. Although some sources, such as Cygnus A, show evidence for a geometrically thick ``torus'' as originally suggested, recent work on the radiation-driven warping instability discovered by Pringle suggests that obscuration by thin, warped disks may play a crucial role in AGN. 
Philip R. Maloney