Global Oscillations of Maser Disks in Active Glactic Nuclei and the Kinematics of Magamaser Sources

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Recent discovery of water megamasers in circumnuclear regions of some type 2 active galactic nuclei (AGN) has opened a new way to probe the innermost parsec of the nuclei. The VLBI observations have revealed the detailed spatial and velocity distribution of the magemaser emission. Maser features in some AGN have been found to arise from geometrically thin, disk-like regions. Since such disks can be perturbed by global m = 1 oscillations, it is important to study the characteristics of these oscillation modes and their effects on the maser source kinematics. For this purpose, we have numerically studied the three dimensional distributions of the density and the velocity field in polytropic disks perturbed by the m =1 eigenmodes. The following is the summary of our results.1. Both z-symmetric (eccentric) modes and z-antisymmetric (warping) modes are present in magemaser disks, where z is the vertical coordinate. The perturbed velocity field associated with the warping mode is approximately vertical, whereas that associated with the eccentric mode is roughly horizontal.2. The model position-velocity diagram and velocity drift for maser disks perturbed by the warping mode are indistinguishable from those of Keplerian disks. This characteristic agrees well with the observed spatial and velocity distribution of the maser features of NGC 4258.3. In maser disks perturbed by the eccentric mode, the position-velocity diagram and the velocity drift become non-Keplerian. The observed sub-Keplerian line-of-sight velocity distribution of NGC 1068 can be attributed to this mode.