Dusty Tori of Seyfert Nuclei Probed by the Water Vapor Maser Emission:   How Large are the Dusty Tori ?
  Geometrical and physical properties of dusty tori of Seyfert nuclei probed by the water vapor maser emission at 22 GHz are discussed. We assume that the dusty torus has a simple cylindrical form and the maser emission can be detected only when we observe the torus from almost edge-on views. The observed low frequency of occurrence of the water vapor maser emission (less than 10 percent) suggests that the torus is a vertically thin cylinder whose outer radius between a few pc and ~ 10 pc. However, the observed masing regions are concentrated in the inner 1 pc regions of the torus. This property can be explained by that only the inner a few pc regions have physical conditions enough to cause the maser emission; the temperature is as high as several hundred K and the density is as high as ~ 10^(10) cm^(-3). 
Yoshiaki Taniguchi, and Takashi Murayama