GAS-DRIVEN EVOLUTION OF STELLAR ORBITS IN BARRED GALAXIES

We carry out a detailed orbit analysis of gravitational potentials sel ected at different times from an evolving self-consistent model galaxy consisting of a two-component disc (stars+gas) and a live halo. The r esults are compared with a pure stellar model, subject to nearly ident ical initial conditions, which are chosen so as to make the models dev elop a large-scale stellar bar. The bars are also subject to hose-pipe (buckling) instability which modifies the vertical structure of the d isc. The diverging morphological evolution of both models is explained in terms of gas radial inflow, the resulting change in the gravitatio nal potential at smaller radii, and the subsequent modification of the main families of orbits, both in and out of the disc plane. We find t hat dynamical instabilities become milder in the presence of the gas c omponent, and that the stability of planar and 3D stellar orbits is st rongly affected by the related changes in the potential - both are des tabilized, with the gas accumulation at the centre. This is reflected in the overall lower amplitude of the bar mode and in the substantial weakening of the bar, which appears to be a gradual process. The verti cal buckling of the bar is much less pronounced and the characteristic peanut shape of the galactic bulge almost disappears when there is a substantial gas inflow towards the centre. Milder instability results in a smaller bulge, the basic parameters of which are in agreement wit h observations. We also find that the overall evolution in the model w ith a gas component is accelerated because of the larger central mass concentration and the resulting decrease in the characteristic dynamic al time.