Gaia DR2 halo and disk projects

Today Kathryn Johnston (Columbia) came through Flatiron to discuss Gaia DR2 projects in the Milky-Way halo. She made the very nice point that we could use a Gaia simulator like PyGaia to “observe” the Bullock & Johnston all-substructure simulations to see how halo substructure appears in a realistic DR2 data set. We discussed clustering algorithms and the relationships between applying clustering directly to the observed data vs transforming the data to some better space (invariants, say) vs doing some kind of inference or data-driven model that respects the Gaia noise model and so on. We are looking for methods that will be powerful, but simple, since we are looking for fast projects to do in the immediate follow-up period to the data release.

Our conversation veered into chemical-abundance space, where we all realized that Megan Bedell (Flatiron) is sitting on an amazing chemical-tagging data set. She only has 80 stars, but because they are Solar twins, they have exceedingly good chemical measurements. Can we use these to measure scattering processes in the Milky-Way disk?

We also briefly discussed something inspired by Alyssa Goodman (Harvard), who spoke first thing in the morning at the Scientific Visualization conference that is on at Flatiron: Can we measure our position relative to the disk plane, and maybe see fluctuations in that plane? Goodman says that the Sun is 25 pc above the plane, and that is obvious (she says) from the radio observations of HI gas. But Bovy (if I recall correctly) looked at this in Gaia DR1 and finds that our offset from the midplane is less than 10 pc. Is there an offset between stars and gas? If so, why? If not, who is wrong? Great set of questions for DR2.

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