SMOOTHED PARTICLE HYDRODYNAMIC SIMULATIONS OF GALACTIC GASEOUS DISK WITH BAR: DISTRIBUTION AND KINEMATIC STRUCTURE OF MOLECULAR CLOUDS TOWARD THE GALACTIC CENTER

摘要

We have performed smoothed particle hydrodynamic (SPH) simulations to study the response of molecular clouds in the Galactic disk to a rotating bar and their subsequent evolution in the Galactic center (GC) region. The Galactic potential in our models is contributed by three axisymmetric com- ponents (massive halo, exponential disk, compact bulge) and a nonaxisymmetric bar. These components are assumed to be invariant in time in the frame corotating with the bar. Some noticeable features such as an elliptical outer ring, spiral arms, a gas-depletion region, and a central concentration have been developed due to the influence of the bar. The rotating bar induces noncircular motions of the SPH particles, but hydrodynamic collisions tend to suppress the random components of the velocity. The velocity field of the SPH particles is consistent with the kinematics of molecular clouds observed in HCN (1-0) transition, these clouds are thought to be very dense clouds. However, the longitude-velocity (l-v) diagram of the clouds traced by CO is quite different from that of our SPH simulation, being more similar to that obtained from simulations using collisionless particles. The l-v diagram of a mixture of collisional and collisionless particles gives better reproduction of the kinematic structures of the GC clouds observed in the CO line. The fact that the kinematics of HCN clouds can be reproduced by the SPH particles suggests that the dense clouds in the GC are formed via cloud collisions induced by the rotating bar.