At the Gaia Sprint, there is no formal program. It is just work, work, and more work! But we do let the participants self-organize some break-out sessions that are more like sessions in a (highly interactive) workshop. Today, we ran a session on a possible ESA Gaia DR2 selection function. There is no selection function, and this seriously limits the science we can do with the mission and its data. I opened the session with some generalities about what a selection function should or could be and how we would use it, working from notes that Rix (MPIA) and I have been working on. I learned that we are describing it all wrong, and that we need much better and more worked-out example problems. It is very interesting to classify projects into those that do and those that don't need a selection function. Rix and I put it on our to-do list to re-work our paper outline on this.
In the sprinting part of the day, Eilers (MPIA) and I stepped back and realized that we should make all nine obvious kinematic plots of the Milky Way disk: Mean velocity (three plots), mean squared velocity (three plots) and mean velocity-velocity cross-correlation components (three plots). We started on that, and the bar looks like it just pops right out in the plot of the mean-square vertical velocity component! We are starting to realize that the things we want to plot that relate to the bar are very different from the things we want to plot that relate to the spiral arms.
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