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N.F. Comins, T.M. Zeltwanger (University of Maine, Orono, ME, 04469)
Our two dimensional simulation of disk galaxies has five components: collisionless N-body particles representing star clusters; colliding N-body particles representing giant molecular clouds (GMCs); a gravitating hydrodynamic component simulating the intercloud medium; a dark-matter halo; and, a central black hole. Besides gravitational interaction between all the components, we simulate a high degree of evolution: stars in each cluster evolve, supernova, return mass to the intercloud medium; new GMCs form from Jeans unstable gas; GMCs collide, coalesce, fragment, or scatter, depending on physical conditions of their interactions; new star clusters form from the clouds in a variety of ways; and mass falls onto the disk from the halo.
Using 100,000 star particles, 10,000 clouds, 75% halo mass fraction, and full evolutionary activity, we are able to simulate persistent spiral structure that lasts for at least 44 rotation periods or 11 Byr. Runs with identical mass distributions, but no evolution, show no spiral structure.