for
0.57pc < r < 0.92pc.
A.T. Okazaki
Faculty of Engineering, Hokkai-Gakuen University
Toyohira-ku, Sapporo 062-8605, Japan
NGC 1068 is considered to be the archetypal galaxy, harboring an active galactic
nucleus heavily obscured by a dense molecular torus.
In the nucleus of NGC 1068 lies a megamaser source which enables us to probe
the structure and the velocity field of the innermost part of the torus. Recently,
Greenhill et al. (1996) observed the water maser emission in NGC 1068 with
the VLBA and obtained images with sub-milliarcsecond angular resolution. Their
best fit rotation curve for the redshifted maser emission was sub-Keplerian;
the line-of-sight velocity of the maser source decreased as for
0.57pc < r < 0.92pc.
In this paper, we examined the effect of global oscillation modes on the kinematics of the maser emission region of NGC 1068. For this purpose, we used a simplified disk model and solved the resulting eigenvalue problem in the three dimensions. We found that there exist global eccentric (z-symmetric m=1) modes as well as warping (z-antisymmetric m=1) modes in the masing disk of NGC 1068. Figure 1 shows the three-dimensional structure of the fundamental eccentric mode.
The global eccentric mode can explain the observed sub-Keplerian velocity
distribution of the maser source of NGC 1068. Figure 2 shows the
position-velocity diagram calculated under the assumption that the masing disk
is perturbed by the fundamental eccentric mode. As shown in Figure 2, the
eccentric mode flattens the velocity gradient of the redshifted emission and
shifts the systemic velocity blueward by 
. Our model
predicts that future VLBA observation of the blueshifted maser emission of
NGC 1068 will reveal that its line-of-sight velocity distribution is steeper
than the Keplerian distribution.
