Detection of a water-vapor megamaser in the active galaxy NGC 5793

We have found an extragalactic water-vapor megamaser source in NGC 5793, an edge-on spiral galaxy. Its spectral profile shows two distinct high-velocity maser features, one on each side of the systemic velocity of the galaxy. They are almost symmetrically offset from the systemic velocity. The isotropic luminosity of the features is as large as 125 L. at a distance of 50 Mpc. Weaker features at the systemic velocity were possibly detected. We have constructed a model which explains our observational data. The H2O maser emission is emitted from a nearly edge-on circumnuclear disk, or torus; whose diameter is estimated to be larger than 4-20 pc: with a rotation velocity of V-rot = 245 km s(-1) at an inferred inclination angle of 73 degrees. The high-velocity features arise from gas at the tangential regions of the disk, or torus, and the systemic features from the region in our line of sight toward the center. In our model, the nuclear mass confined within the disk or torus is larger than 1.5 x 10(8) M..