My main research today was a long call with Suroor Gandhi (NYU) about papers that determine the dark-matter density in the local part of the Milky Way disk by modeling the stars as an equilibrium population. The idea is that if the population is in equilibrium, it has some properties (like obeying the Jeans equation) that permit it to be used to do inference of the potential. I don't love these papers, both because of the assumptions they make (reaching equilibrium takes a very long time) and because of the ways that they boil down the data to some summary statistics before doing inference. Can't we generate the data and write down a proper likelihood? But more importantly, can't we do inference on non-equilibrium problems? I think we can! That's why Gandhi and I are looking at The Snail (the phase spiral). I think it reveals the orbit structure of the disk pretty-much directly, and we ought to be able to beat the equilibrium models both in precision (measuring orbits is better than measuring velocity moments) and in parsimony (we don't have to make assumptions that are as strong).
Howdy! I agree with your point about the assumption of equilibrium, which illustrates why it's critical to check the assumption in whatever ways are available. Speaking for our analysis, we did these checks in every way we could think of and found that our sample of tracer stars matched all the criteria for a system in equilibrium to the extent that the data could tell. Certainly there have been others who have found evidence of disequilibrium, but this is typically manifest in different populations and with larger spatial volumes (near the midplane the equilibration timescale is considerably shorter). To the extent that we could, we tried to estimate systematic uncertainty coming from disequilibrium and found that it didn't really affect our results. At some point, if it quacks like a duck...
ReplyDeleteReally glad to see somebody doing a complementary analysis though! It'll be great to close all possible loopholes