After playing with visualization yesterday, Christina Eilers (MPIA) and I got the idea that perhaps the radial-velocity variations we see in the Milky Way disk might indicate density variations. In particular, does the radial-velocity field converge on high-density regions in the disk (spiral arms, say) and diverge on low-density regions (inter-arm gaps, say)? Sarah Pearson (Flatiron) came to our rescue with a nice visualization of the density and velocity fields, in which she could smoothly go from showing one to the other. And indeed, our intuitions were justified, at least qualitatively.
In the evening check-in, Paolo Tanga (Côte d'Azur) showed some beautiful results on the ESA Gaia coordinate systems relative to other catalogs. He calls these differences "zonal corrections" for historical reasons! I asked him how he knows which of the coordinate systems is best, and he said: In the best frame, the asteroids will travel on calculable trajectories. (I would say gravitational trajectories, but for asteroids, radiation pressure and other forces are relevant too!) So the best coordinate system will be Newtonian in the Solar System! Of course given frame dragging, and strictly speaking, Newtonian for the Solar System will not be Newtonian for the Galaxy! I asked about that and it led to some discussion with Larry Widrow (Queen's). I have much to say about all this, but I'm not yet ready to say it out loud.
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