spiral arms? and model-grid troubleshooting

The excitement of the day is that we looked at velocity-tensor maps (maps of the means of average velocity-velocity products) across the disk with Eilers (MPIA): We see lots of structure, including possible evidence of spiral arms or bar resonances on the off-diagonal tensor components. Reminder: If the Galaxy is axisymmetric, there will only be diagonal tensor components in the R, phi, z coordinate system. If we find off-diagonal components: Non-axisymmetry. Could be interesting. Rix (MPIA) encouraged us to stay on target for a Jeans model and leave these hints of complex disk morphology for later investigations.

In addition to this, I had a great chat with Maria Bergemann (MPIA) and Mikhail Kovalev (MPIA) about fitting spectra with spectral models, given that the models are amazingly expensive to compute. They do a (random) grid and then interpolate using The Payne. They are getting some results they aren't happy with, so I walked through basic tests that can be done in these situations.

Basic sanity checks—when you are fitting data using an interpolation of a grid or random assemblage of model predictions—are the following: Find the closest model point in the grid, and then the K next closest, where K is larger than the dimensionality of the model parameter space. Is the best-fit model in the convex hull of the K? Are the K in one group or multiple groups? Do the K look like they hit the edge of the grid? And what are the chi-squared values? And is the interpolated best point also in the convex hull? All these pieces of information go into an analysis of whether you have enough model evaluations and how to interpolate them.

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