ELSA Conference on Gaia, day five

This morning was the final session of the meeting. Again, there were too many things to mention. Helmi (Groeningen) began the day with a discussion of streams and substructure. Among other things, she showed that substructure falls onto the Galaxy along preferred directions, so features like the famous bifurcation in the Sagittarius stream is not unlikely caused by a pair of galaxies infalling along the same plane! She suggests looking for streams in the 3-dimensional space of actions; this is a good idea, but a young stream also is compact (1-dimensional) even in the action space, so there is more information than just what you find in action space.

At the end of the meeting, I gave my talk on catalogs as models and the idea of propagating likelihood information about image intensity measurements rather than a hard catalog. I realized during the question period that this is my main problem with the semantic ideas in astronomy, and that should be a part of my write-up. Brown (Leiden) followed with a concluding remarks talk in which he was extremely nice to me, but also advocated releasing Gaia results early and often, and releasing the results in such a way that they could be re-analyzed at the pixel level in the future. I wholeheartedly agree. Farewell to a wonderful meeting.