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 that 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%) suggests that the torus is a vertically thin cylinder whose outer radius between a few parsecs and ~10 pc. However, the observed masing regions are concentrated in the inner 1 pc regions of the torus. A possible explanation for this property is 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 ~1010 cm-3.