[As a result of crippling disorganization, blog posting has gone pear-shaped. I will be posting out-of-date posts with correct dates (and 23:59 time stamps), but possibly considerably after the fact and out of chronological order.]
Another great day at Ringberg Castle today. For me, the highlight was an unconference session called Gaia Zero-Day Exploit, which was about what we could do immediately upon release of Gaia data in its first and second releases. So many good ideas came up, and most of them are summarized in incredibly telegraphic form on the crowd-sourced meeting minutes (search down the document for "Shovel-ready"). Gaia DR2 is much more interesting than DR1 for Galactic dynamics. Some great ideas included: Find the alleged fountain of hyper-velocity stars coming from the Galactic Center and use them to infer the shape of the MW halo. And: Look at the vertical velocities in the disk and see the pattern speed of disk warping and precession (see mention below of Widrow). And: Look for direct evidence of the process of dynamical friction—that is, look for the stellar wakes. And, simply: Get the proper motions of all Milky Way companions and streams. The list is long. After the session, Rix and I felt like we should find some way to get some of these fleshed out and quasi-published in some community forum.
David Martinez-Delgado (Heidelberg) kicked off the day with his absolutely magnificent images of galaxies, in which he uses small telescopes and long exposure times to get amazingly sensitive images of tidal features. He compared similar images from his small telescopes to those taken with huge telescopes, and showed that smaller is better. There was some discussion afterwards of "why". Some of it is that his calibration requirements are high, and he spent more time on his own instruments (he may be the best flat-fielder in all the land). But some of it is that big telescopes have big cameras with many optical surfaces, which make it hard to go for large dynamic range in intensity.
Larry Widrow (Queen's) talked about galactic plane distortions and coined the term "galactoseismology".
After watching various talks in which streams were simulated, Price-Whelan and I noticed that for the first few orbits (even in chaotic potentials), the stream stars make a "bowtie" shape when very near apocenter. And the stars spend a lot of time out there. So perhaps we should be looking for these: There might be many more bowties than streams! We discussed.