Spiral patterns are one of the most important agents of dynamical evolution in disk galaxies. They redistribute angular momentum, cause the random speeds of stars to rise over time, cause radial mixing and large-scale turbulence, add stars to the thick disk, and flatten rotation curves. This project is a computational study of the effects of spiral patterns on disk evolution. The Prinicpal Investigator (PI) has several well-developed gravitational N-body codes that he will use with supported graduate students in two projects: (1) The Milky Way's thick disk is composed of old stars, in conflict with the "spiral churning" model for the formation of this component. The PI hypothesizes that the appearance of a bar in the inner Milky Way may have shut off thick disk growth by "churning." He plans simulations to test this idea, comparing the evolution of similar disks in which bars are or are not formed. (2) It is still ambiguous how spiral galaxies manage the so-called "disk-halo conspiracy," in which the different contributions of the disk and dark halo gravitational fields produce a flat observed rotation curve in the observable stars and gas. Simple models by the PI and collaborators showed that spiral waves could cause the mass distribution in a disk to rearrange itself until the rotation curve became flat; he plans further simulations including "live" halos to verify the generality of this behavior. Finally, the PI will continue work on recurring spiral patterns and their observable consequences in local stellar population. The PI plans careful N-body simulations of unprecedented quality for an in-depth study that will uncover the details of the recurrence mechanism in simplified models. Graduate students will assist with projects to show that the behavior persists when simplifying assumptions are relaxed. The PI will disseminate results of this work to the broader community through public lectures. He will involve undergraduate and graduate students, and postdocs, in the research, providing them with experience and training in the research process.
