Formation and Fragmentation of Gaseous Spurs in Spiral Galaxies

We believe that intermediate-scale spur structures, seen most clearly in the Whirpool galaxy M51, may arise from self-gravitating instabilities in magnetized spiral arms; the increased shear in interarm regions leads to the ``backswept'' morphology. The enhanced densities within spurs makes possible smaller-scale fragmentation, which may produce bound clouds of a few million solar masses.

            The Whirpool Galaxy

     Simulated dynamics in spiral-arm  
     segment (Q=1.5, beta=1)