Two modes of gas flow in a single barred galaxy

We investigate stationary gas flows in a fixed, rotating barred potential. The gas is assumed to be isothermal with an effective sound speed cs, and the equations of motion are solved with smoothed particle hydrodynamics (SPH). Since the thermal energy in cloud random motions is negligible compared to the orbital kinetic energy, no dependence of the flow on cs is expected. However, this is not the case when shocks are involved. For low values of cs an open, off-axis shock flow forms that is characteristic for potentials with an inner Lindblad resonance (ILR). Through this shock the gas streams inwards from x1 to x2-orbits. At high sound speeds the gas arranges itself in a different, on-axis shock flow pattern. In this case, there is no gas on x2-orbits, demonstrating that the gas can behave as if there were no ILR. The critical effective sound speed dividing the two regimes is in the range of values observed in the Milky Way. We give a heuristic explanation for this effect. A possible consequence is that star formation may change the structure of the flow by which it was initiated. Low-mass galaxies should predominantly be in the on-axis regime. A brief comparison of our SPH results with those from a grid-based hydrodynamic code is also given.