Gaseous vortices in barred spiral galaxies

Simulations of the gas how in a variety of two-dimensional barred spiral ga laxies have shown that vortices in the gas appear, when viewed from above, in the corotation frame of the bar. These low-density vortices generally ap pear at or near the L4 and L5 Lagrangian points. We show that these gas vor tices in our models are the hydrodynamic analogs of closed, long-period orb its centered on L4 and L5. Secondary but high-density vortices can appear a long the L1-L2 axis. The presence of the vortices at or near L4 and L5 lead s to a possible practical application, namely, the determination of the cor otation radius. Our models have shown that, when viewed in the rotating fra me of the perturbation, vortices are present at or very dose to corotation and with position angles approximately 90 degrees with respect to the pertu rbation. As the viewing frame angular velocity is changed, both the radial positions and the position angles of the vortices change. If a gas-rich, barred spiral galaxy were observed in H I with sufficient re solution and signal-to-noise ratio, and if the gas velocities in that regio n are transformed so that they are "observed" in a rigidly rotating frame, having its origin at the galaxy's center, a pair of vortices should appear with position angles approximately 90 degrees with respect to the bar when the angular velocity of the observing frame equals the pattern speed of the bar. Based upon these simulations, we estimate that the pattern speed and corotation radius can be deduced to within approximately 5%-25% of their tr ue values depending upon the quality of the observations.