Long-Lived Spiral Structure for Galaxies with Intermediate Size Bulges
  Spiral structure in disk galaxies is modeled with nine collisionless N-body simulations including live disks, halos, and bulges with a range of masses. Two of these simulations make long-lasting and strong two-arm spiral wave modes that last for $\sim5$ Gyr with constant pattern speed. These two had a light stellar disk and the largest values of the Toomre $Q$ parameter in the inner region at the time the spirals formed, suggesting the presence of a Q-barrier to wave propagation resulting from the bulge. The relative bulge mass in these cases is about 10\%. Models with weak two-arm spirals had pattern speeds that followed the radial dependence of the Inner Lindblad Resonance. 
Kanak Saha and Bruce Elmegreen
astro-ph.GA