4.2. Gas flow in barred galaxies
In summary of the previous section, the orientation of the periodic orbits rotates by 90° at each resonance crossing, and they are successively parallel and perpendicular to the bar. The gas will first tend to follow these orbits, but the streamlines of gas cannot cross. Since periodic orbits do cross, gas clouds can encounter enhanced collisions, such that their orbits are changed. Instead of experiencing sudden 90° turns, their orbits will smoothly and gradually turn, following the schematic diagram of kinematic waves, first drawn by Kalnajs (1973), and illustrated in fig 8 and 9.
This interpretation predicts that the arms will be more wound when there exist more resonances; there will be a winding over 180° with only CR and OLR, with the gas aligned with the bar until corotation. When there exists 2 ILRs, the gas response can be perpendicular to the stellar bar. When there is barely one ILR, strong shocks can occur on the leading edge of the bar, corresponding to the offset dust lanes observed in barred galaxies (cf fig 5).
Figure 5. Response of the gas to a bar potential. The left panel shows the gas density in grey-scale and the right panel the gas flow lines and the velocity vectors in the rotating frame of the bar. The bar potential is at 45° from horizontal (from Athanassoula, 1992).