WOULD A GALACTIC BAR DESTROY THE GLOBULAR-CLUSTER SYSTEM

Noncircular motions at 3 kpc in our Galaxy have been interpreted, kinematically, as streamlines in a central bar. We construct dynamical models for the Galactic potential satisfying the observed rotation curve but containing a central bar so that the 3 kpc nonintersecting streamlines have a radial velocity of 50 km s-1 when viewed at 45-degrees to the bar axis. Five different Galaxy models are used: two axisymmetric control models and three barred models: a nonrotating bar of mass 0.8 times the total disk mass, a nonrotating bar of 0.25 times the disk mass, and a rotating bar of mass 0.25 times the disk mass. The rotating bar has an angular frequency OMEGA(b) = 40 km s-1 kpc-1, reaching corotation at 5 kpc. All of the bars have an axis ratio of 5:1 and scale length 2.5 kpc. The rotating bar and the larger nonrotating bar are able to mimic the 50 km s-1 noncircular motion observed at 3 kpc. In the potential of the small rotating bar, the streamlines are aligned with the bar, but there is a region between 3 and 6 kpc where there are no closed non-self-intersecting streamlines, and so little gas is expected. The existence of a large nonrotating bar can be ruled out because the inner streamlines are (unphysically) orthogonal to the bar axis. We then investigate the effect of these central bars on the destruction rates of globular clusters in our Galaxy. Since some fraction of the orbits in a barred Galaxy will be boxes, and these will ultimately pass arbitrarily close to the Galatic center, it is reasonable to expect that the presence of a bar will increase the destruction rates due to bulge shocks. If the destruction rate of globular clusters is very drastically increased by the presence of a bar, it may be possible to rule out the existence of a bar. We apply the method of Aguilar, Hut, & Ostriker to our barred Galaxy models. The unknown tangential velocity components of each observed cluster are drawn randomly from an assumed distribution function. The cluster's orbit is integrated, and the bulge shocking rate is calculated. The median destruction rate of the cluster is computed by sampling a large number of such orbits. The addition of the rotating bar does not strongly affect the destruction rates of globular clusters. There is a small increase in the destruction rate for those clusters within about 2.5 kpc. Thus it is not possible to rule out the existence of a rotating bar on these grounds. In addition, the existence of the gap in the case of a rotating bar is similar to that seen in the gas density of our Galaxy, though in our model it is in the wrong place. We conclude that this study is consistent with the hypothesis that our Galaxy contains a small (major axis approximately 3 kpc) rotating bar at its center.