Today we had a presentation from Axel Widmark (NBI), who has traced the spiral structure in the Snail (tm), which is the phase spiral in the vertical dynamics discovered by Antoja and others in the ESA Gaia data. He uses the trace to estimate the density of the Milky Way disk. Widmark's approach reminds me of old-school work on stream tracks (by, for instance, Johnston and others, but also maybe me?): He traces the spiral ridge-line and asks for it to represent a frequency dependence on vertical energy. That's a good idea! And it seems to work. And it motivated me to think more about methods that—rather than just trace a linear locus—perform full distribution-function fitting in a time-independent potential.
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