spiral structure

This morning on my weekly call with Eilers (MPIA) we discussed the new scope of a paper about spiral and bar structure in the Milky Way disk. Back at the Gaia Sprint, we thought we had a big result: We thought we would be able to infer the locations of the spiral-arm over-densities from the velocity field. But it turned out that our simple picture was wrong (and in retrospect, it is obvious that it was). But Eilers has made beautiful visualizations of disk simulations by Tobias Buck (AIP), who shows very similar velocity structure and for which we know the truth about the density structure. These visualizations say that there are relationships between the velocity structure and the density structure, but that it evolves. We tried to write a sensible scope for the paper in this new context. There is still good science to do, because the structure we see is novel and spans much of the disk.