Speaker
Description
The Gaia Snail—a spiral pattern in stellar phase space caused by a perturbation to the Milky Way disk—is a natural laboratory for one-dimensional collisionless gravitational dynamics, the sign-flipped analog of electrostatic Vlasov–Poisson physics. Linear theory reproduces the spiral but winds it too tightly and fails to reproduce salient observed features. We present nonlinear analytical results and Gkeyll simulation verification for this gravitational cousin of classic plasma kinetic problems. The self-gravity operator around the classic Spitzer equilibrium, which is a reflectionless soliton potential, produces an exactly zero first-order shift. The quasi-linear second-order shift is shown to be negative. Self-gravity decelerates phase mixing. The winding power spectrum is exactly Gaussian in action space, computed by a technique due to Lundgren from a vortex spiral model of fluid turbulence. The nonlinear endpoint is a BGK quasi-stationary state, approached through a free-energy cascade in action space. The relevant statistical mechanics is not Lynden-Bell violent relaxation but phase-mixing entropy production: fine-grained Casimir invariants are conserved while coarse-grained power cascades irreversibly to sub-resolution scales.