Dusty Tori of Seyfert Nuclei Probed by the Water Vapor Maser Emission: How Large Are the Dusty

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 cm−3. Subject headings: galaxies: active galaxies: Seyfert ISM: molecules masers radio lines: galaxies infrared: emission