The response of gas in a galactic disk to bar forcing

The response of a gaseous disk to a rotating stellar bar is investigated. The gas is simulated by 2000 particles or clouds, which are allowed to collide and in doing so can dissipate a large fraction of their relative motion. The response in a model with no inner Lindblad resonance consists of a trailing spiral shock extending from corotation to just outside the outer Lindblad resonance (OLR), and can be interpreted as a result of the intersection of periodic orbits. The pattern evolves slowly toward a ring which is made up of a narrow range of periodic orbits belonging to one of two families. The ring at the OLR is usually aligned with the bar, although the parameters of the model can be so arranged that the ring forms perpendicular to the bar. Replenishment of the gas at a rate 1-2 solar masses/yr through stellar mass loss or infall onto the disk can preserve the spiral structure indefinitely. The models bear remarkable resemblance to many real galaxies with regard to gross features in both the hydrogen distribution and its velocity field.