Aspen, day 1

I'm in Aspen for the week, working on Milky Way dynamics and chemical abundances. The day started with introductions, in which many themes arose. One is that detailed abundances are here, are good, are numerous, and are under-utilized. So the work I am doing fits in pretty well! It occurred to me (Duh!), when some of the more particle-oriented people spoke, that imaging the dark matter is no different from testing gravity, at least conceptually. So I can spin off a testing-gravity side project.

The MCMC runs I sent off working on my laptop on Friday did not disappoint: I got amazingly strong constraints on the dynamics of the disk, including a percent-level measurement of the disk density! That's a precision of course; at percent level none of my assumptions are defensible. But it works really well: The iso-chemical contours really do show you the orbits, and precisely.

But, the most interesting respect in which my assumptions are violated is that the different elements want to put the midplane of the disk in different locations! Huh? And the effect is just clearly visible in the GALAH data. Rix (MPIA) pointed out today that the phase-mixing times can be long near the disk center because in the limit of a harmonic potential, all frequencies are degenerate and mixing doesn't happen. So maybe we have clear evidence for non-phase-mixing, vertically, in the disk. Or of course maybe there are (very adversarial, I might say) issues with the GALAH data. But the nice thing is you can just see it in the element abundances. Look for yourself: The midplane in [Fe/H] looks different from the midplane in [Si/Fe]. (Plots have z-velocity on the horizontal axis and z-position on the vertical axis).