THE EXISTENCE AND SHAPES OF DUST LANES IN GALACTIC BARS

I study the flow of gas in and around the bars or ovals of model barred galaxies and show that it is intimately linked to the properties of the periodic orbits. Simulations show that the density of the gas in and around the bar region is low, except for the centre and two narrow lanes which are the loci of shocks. Such shocks form if the x1 periodic orbits have either loops or large curvature values at their apocentres. The form of the shock loci depends on a number of parameters characterizing the bar and disc potentials. In order for shocks to be offset from the bar major axis towards its leading side, the x2 and x3 families must not only exist but also cover a sufficient extent along the bar major axis. The shapes of the shock loci are then understood by the gradual shift of the orientation of the flow lines from that of the x1 orbits to that of x2. Along the shock there is also substantial shear, opposing star formation. The existence of offset shock loci of the observed shape puts strict constraints on the values the various model parameters may take. This should be most useful for deriving bar parameters for observed galaxies. For example, the Lagrangian radius has to lie in the interval (1.2 +/- 0.2) a, where a is the length of the bar semimajor axis. For low values of the bar axial ratio or the bar quadrupole moment the shock loci are curved, with their concave sides towards the bar major axis. Substantial inflow is found in models with strong shocks.