Speaker:  Curtis Struck, Professor Emeritus

Title:  The how and why of exponential surface density profiles in galaxy disks

Abstract:  Exponential radial profiles are ubiquitous in galaxy disks; they form early and evidently are reestablished promptly following major disturbances. A number of different processes have been proposed to account for this profile form, including: primordial collapse, viscous evolution, resonant migration and scattering by clumps, spiral waves, bars and companions. These processes may work on different timescales, in parallel, but resonant migration and direct scattering are likely to be the most important for most stellar disks. Scattering by a bar may be very important in barred disks, including the Milky Way. Young disks are very different from present day disks. Massive clumps are common, global spirals and bars much less so. We have recently published extensive N-body simulations of clump scattering in stellar disks, as well as analytic and Monte Carlo calculations. Disks with arbitrary initial profiles evolved to an exponential profile in all cases, but on timescales that depended on the clump mass distribution. Scattering was generally biased and the strongest scattering events result from transient alignments of clumps. It was also found that very eccentric orbits can partially re-circularize. A steady stellar orbital eccentricity distribution develops, rather than a trend toward increasingly radial orbits. Ongoing work explores the scattering effects of companions and waves.