31 March 2011

Speaker: Lars Hernquist (Harvard / CfA)

Title:Collective Origin of Spiral Structure in Disk Galaxies

Abstract:It is now nearly 50 years since spiral structure in galaxies was hypothesized to originate from density waves propagating through a shearing disk. However, the nature of this process remains uncertain. Relevant theories range from interpreting spiral arms as long-lived density waves to their being produced stochastically in response to gravitational perturbations. In this talk, I examine the latter possibility, where spiral arms are seeded by density fluctuations orbiting within a disk. Using high-resolution simulations, I study the response of a thin, differentially rotating disk of stars to a population of perturbers. Individually, each perturber excites a wake locally in the distribution of stars around it. When sufficient numbers of these perturbers are present, they collectively amplify to yield large-scale patterns that resemble those in flocculent and intermediate spiral galaxies. Combining the N-body experiments with simple analytic arguments, I develop a theory for spiral structure based on the collective effects of swing amplification. The model makes numerous testable predictions, making it possible to finally test the theory that spiral arms are stochastic in nature.

Video of the Presentation (Talks can be viewed with RealPlayer. Free download is available from www.real.com )