streams of stars in the Milky Way

Bonaca (Yale), Geha (Yale), Johnston (Columbia), Kuepper (Columbia), and Price-Whelan all came to NYU to visit CampHogg to discuss stream-fitting. We (like everyone on Earth, apparently) want to use streams to constrain the potential and accretion history of the Milky Way. Kuepper and Bonaca are working on simulation methods to make fake streams (quickly) and compare them to data. Price-Whelan is working out a fully probabilistic approach to generating stream data with every star carrying a latent variable which is the time at which it was released from the progenitor (this is my "Bread and Butter" project started at the end of this summer after sessions with Binney, Bovy, Rix, Sanders, and Sanderson). We have hopes of getting good inferences about the Milky Way potential (or acceleration field or mass density) and its evolution with time.

Price-Whelan and Foreman-Mackey spent some time coming up with very clever Gibbs-like strategies for sampling the per-star latent parameters (release time and orbit for each star) in an inner loop with an outer loop sampling the potential and progenitor parameters that are shared by all stars. In the end, we decided to de-scope and write a paper with brute-force sampling and a small data set. Even at small scope, such a paper (and software) would be state-of-the-art, because what we are doing treats properly missing data and finite observational uncertainties, which will be a first (unless Bovy or Sanders has scooped us?).

At lunch, I asked the team to say what a stream really constrains: Is it the potential, or the acceleration field, or the density? Clarity on this could be useful for guiding methods, expansions, parameterizations, and so on. In the afternoon, Geha, Johnston, and I also talked about joint funding opportunities and outlined a proposal.