Gas response to oval distortions in disk galaxies.

Numerical hydrodynamic calculations have been carried out in order to investigate the response of a non-self-gravitating gaseous component to a rotating weak oval distortion in the axisymmetric gravitational field of a disk. The purpose of these time-dependent calculations was to determine the form of the steady-state gas density and flow distributions which would develop in the rotating frame of the oval distortion. The calculations reveal that the gas responds to the presence of the oval distortion by forming an open two-arm trailing spiral wave rotating with the angular speed of the oval distortion. The gas flow pattern is similar in form to predicted gas flow through small pitch-angle gravitational self-consistent spiral waves, although the deviations from uniform circular motion are large (approximately 75 km/s in regions of low gas density). These results may be understood from an investigation of periodic particle orbits in the rotating frame of the bar. Such an investigation implies that the presence of all three resonance regions (inner Lindblad, corotation, and outer Lindblad) within the gaseous disk is a necessary condition for such a spiral response.