On the weekend and today, Eilers (MPIA), Rix (MPIA), and I started to build a true ansatz for a m&equals2 spiral in the Milky Way disk, in both density and velocity. The idea is to compute the model as a perturbation away from an equilibrium model, and not self-consistent (because the stars we are using as tracers don't dominate the density of the spiral perturbation). This caused us to write down a whole bunch of functions and derivatives and start to plug them into the first-order expansion away from the steady-state equilibrium of an exponential disk (the Schwarzschild distribution, apparently). We don't have an ansatz yet that permits us to solve the equations, but it feels very very close. The idea behind this project is to use the velocity structure we see in the disk to infer the amplitude (at least) of the spiral density structure, and then compare to what's expected in (say) simulations or theory. Why not just observe the amplitude directly? Because that's harder, given selection effects (like dust).