#LGAstat, day 2

Today was day two of the ego-boosting Local-Group Astrostatistics meeting (#LGAstat). It was another great day of talks, some (personal, idiosyncratic) highlights of which follow:

Bird (Vanderbilt) convinced us (or nearly so) that there might be remnants of galaxy evolution encoded in the Milky Way disk (even despite radial mixing and so on). He pointed to evidence that disks form "upside down" (oh the horror of that): At high redshift, disk star formation is dynamically hotter than at low redshift. Relatedly, Nidever (Michigan) showed us a bimodality in disk-star metallicities somewhat unlike what Zolotov described for the halo yesterday. It appears everywhere, but with the two modes in different ratios as a function of location in the disk.

My colleague Anna Ho (MPIA) described The Cannon in detail, and impressed the audience. She is building the version of the code that is for public release and building into survey pipelines. She was asked some questions that get at two big issues with our approach: What do we do if the training set is not "like" the test set? and How do we know that when we are determining label X we aren't just learning a relationship between label X and other labels that are physically unrelated but covariant in the training set? On the former, we can't beat reality, but The Cannon does contain a realistic noise model, so we don't need the training and test sets to be the same in signal-to-noise. On the latter, all we can do is look for measures of statistical independence in our predictions. However, I have a deep-seated but unarticulated fear that we could get hosed by a training set with certain kinds of label degeneracies. More to think about there.

Bovy talked about the structure of the Milky Way in abundance slices, and also how to analyze incomplete catalogs. That is, he talked about the likelihood function and inference in the presence of bad incompleteness.

Schlafly (MPIA) and Green (CfA) showed us results from the three-dimensional Milky Way dust map they have built from the PanSTARRS data. Incredibly beautiful! The day ended with Milky Way bulge talks. Ness showed and interpreted Lang's beautiful WISE image of the X-shaped bulge, which has appeared (to date) only on Twitter (tm)! It appears that one side of the X is closer (and larger in angular extent) than the other, which is beautifully consistent with the expectations from that orbit structure.

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