Today was the start of the second annual Not Ready for Gaia workshop, which brings together the Galactic dynamics groups of Binney (Oxford) and Rix. We are in beautiful Oxford, staying at Merton College. I came as part of Rix's party. Many interesting ideas swung by; here are a few highlights for me:
Binney and Posti (Oxford) talked about equilibrium models for the Milky Way disk and halo. Binney claimed that any reasonable axisymmetric models will be well approximated by integrable axisymmetric models and argued strongly for using integrable models, at least as a starting point. Posti gave some examples.
Evans (Cambridge) argued that we should be using far more flexible models for gravitational potentials, arguing for expansions around simple models. He is spending some time finding simple models that support easy-to-calculate orthogonal bases for perturbations. Both he and Binney emphasized that a good expansion is one in which not too many terms are required for good representation of the data. That's a tall order!
I got into a fight with Evans, Belokurov (Cambridge), and Koposov (Cambridge) about their fitting of the Sagittarius stream. They constrain the stream using a few summary statistics of the data, not the full data. I claimed that their model was bad because it couldn't possibly go through all the data! However, by the end of the day they more-or-less convinced me that maybe the model does go close to the data. Still awaiting a very simple-to-make plot!
Schoenrich (Oxford) and Sanders (Oxford) both discussed the extended distribution function in phase space coordinates plus chemical and age coordinates. An argument broke out during Schoenrich's talk about whether you can "deconvolve" the radial migration process to find out the original birth radius of anything, and whether or not that would be useful. I enjoyed that! Rix pointed to evidence from Mitshang and collaborators that chemical tagging might not work; I have comments on that work which I hope to write up soon.
At the end of the day, Schoenrich showed what might be extremely good metallicity determinations using photometry. He can demonstrate precision, but not yet accuracy. Very interesting to watch; he is building on early work by Ivezic back in the day.