Spiral Morphology and Radial Migration: Kinematically heating, cooling, and cold

Transient spiral arms are thought to drive radial redistribution of stars, shaping disk galaxies by modifying their age, chemical, and kinematic distributions over time. However, the physical factors controlling how efficiently spirals move stars remain poorly understood. This paper investigates how spiral arm morphology—specifically the number, pitch angle, lifetime, and radial dependence of the pattern speed—influences orbital redistribution via 'cold torquing' at the corotation resonance.

The authors derive analytic expressions for the maximum radial excursion of stars trapped at corotation, explicitly accounting for spiral morphology. These expressions predict that for a density-wave-like spiral, cold torquing is more efficient when the spiral pattern is more open (larger pitch angle). Tracer-particle simulations in both two- and three-dimensional galactic potentials confirm this prediction.

In contrast, spirals with a radially dependent pattern speed that corotate with the disk at all radii show the opposite trend: cold torquing becomes more efficient as the spiral winds up to smaller pitch angles over time. The study also finds that the same transient spiral that drives cold torquing naturally produces both kinematic heating and cooling of orbits away from corotation.

These results indicate that spiral morphology alone cannot predict the efficiency of cold torquing. They suggest that the relationship between spiral pitch angle and radial redistribution could serve as a diagnostic test to distinguish between competing theories of spiral structure.