ELSA Conference on Gaia, day four

Today I gave Bovy's talk on inferring dynamical properties from kinematic snapshots. The talk noted that in our April Fools' paper, the planets with velocity closest to perpendicular to position do the most work. After, Juric (Harvard) asked if there are similarly important stars for the Milky Way? If so, we could just observe the hell out of those and be done! My only response is that if we had a sample of realizations of the Milky Way that generate the data—that is, detailed dynamical models—the stars whose properties vary the most across the sampling would be the critical stars. Not a very satisfying answer, since that chain would be almost impossible to produce, maybe even by 2020. Interestingly, my attitude about this turns out to be very similar to that of Pfenniger (Geneva), who gave a very nice philosophical talk about how our vision of the simplicity or complexity of the Milky Way has evolved extremely rapidly. He argued that no smooth models are likely to give right answers, not even approximately right. I don't think everyone in the business would agree with that, though I do. Before either of our talks, Juric gave a summary of his three-dimensional Milky Way modeling using photometric data. He, of course, finds that simple models don't work, but he has some great tools to precisely model the substructure.

Ludwig (Heidelberg) argued that either there are systematic issues with chemical abundance indicators or else even single stellar populations have metallicity diversity. This would do damage to the chemical tagging plans. In conversation afterwards, Freeman (ANU) told me that he thinks that the issue is with the models not the populations and that the models will improve or are improved already. I suggested to Freeman an entirely data-driven approach, but I couldn't quite specify it. This is a project for the airplane home.

Pasquato (Brussels) made very clearly the point that if stars have strong surface features that evolve (as when they have highly convective exteriors), there will be astrometric jitter if the star has an angular size that is significant relative to Gaia's angular precision. This is obvious, of course (and I mean that in the best possible way), but it suggests that maybe even parallax needs to be defined like radial velocity was yesterday!

Antoja (Barcelona), Minchev (Strasbourg), and Babusiaux (Paris) gave talks about the influence of the bar and spiral structure on the Galaxy and galaxies in general. Minchev presented his now-famous results on radial mixing, which relate in various ways to things we have been doing with the moving groups. Antoja was more focused on understanding the specific impact of specific spiral arm and bar models on the local distribution functions. Babusiaux nearly convinced me that the bulge of the Milky Way was created entirely from the disk by the action of the (or a) bar. Bars rule! After dinner, Minchev and I found ourselves at a bar.