a steady-state disk perturbation?

At the beginning of the day I discussed steady-state models of non-axisymmetric perturbations in the Milky Way disk with Frankel (MPIA), Rix (MPIA), and Eilers (MIT). Frankel had found some issues with the equations in our recent arXiv paper on this subject! So we discussed and we are modifying our equations to fix. Thank you, Frankel!

But a big conceptual point under discussion: With the exception of some bars, there are no truly global, steady-state, non-axisymmetric disk perturbations! When you model a perturbation as steady-state, you are really just saying that in some patch (or some annulus), there is an approximately steady-state solution, or that the pattern is changing more slowly than relevant dynamical times. That means that although we use a global pattern in our paper, really anything we find or derive is only locally true.