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Register a new userTwo modes of gas flow in a single barred galaxy
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We investigate stationary gas flows in a fixed, rotating barred potential. The gas is assumed to be isothermal with an effective sound speed c_s, 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 c_s is expected. However, this is not the case when shocks are involved. For low values of c_s 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 x_1 to x_2-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 x_2-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.
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We use a Cartesian grid to simulate the flow of gas in a barred Galactic potential and investigate the effects of varying the sound speed in the gas and the resolution of the grid. For all sound speeds and resolutions, streamlines closely follow closed orbits at large and small radii. At intermediate radii shocks arise and the streamlines shift between two families of closed orbits. The point at which the shocks appear and the streamlines shift between orbit families depends strongly on sound speed and resolution. For sufficiently large values of these two parameters, the transfer happens at the cusped orbit as hypothesised by Binney et al. over two decades ago. For sufficiently high resolutions the flow downstream of the shocks becomes unsteady. If this unsteadiness is physical, as appears to be the case, it provides a promising explanation for the asymmetry in the observed distribution of CO.
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