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Transient Spiral Arms in Isothermal Stellar Systems

We begin by recalling the isothermal, collisionless, disc-halo. The disc component is the Mestel disc. Subsequently we introduce spiral arms to such an isothermal disc-halo system that are co-moving in the mean with an axi-symmetric background. These correspond to a similar disturbance in the halo, which is comprised of spiral structures on cones. The arms are necessarily transient due to the differential winding in the disc and their gradual destruction is described. Although the spiral potentials are weak compared to the axi-symmetric potential the arms are not propagating waves on the background, but rather co-move with it. They have an effect disproportionate to their relative magnitude on the gas distribution in the disc. The gas accumulates on the outside leading edge of the 'stellar' arm and an arm-interarm modulation of up to 100% is possible. Compatible isothermal, scale-free, distribution functions are found either exactly or approximately for all of the collisionless components of the disc-halo system. Repeated episodes of winding arms can produce an exponential disc.