#LGAstat, day 3

Today was a great day at #LGAstat. Here is a personal, non-exhaustive set of highlights:

Roth (UCL) gave a great talk about understanding galaxy or galaxy-group properties as a function of cosmological initial conditions. She uses a framework in which she can adjust the initial conditions draw (to, say, have a certain total density, or a ratio of density in one part relative to another, or etc) and see how the simulation results depend on the changes to the initial conditions. Her method permits her to move the ICs smoothly, but also ensure that they remain quantitatively comprehensible as a draw from the true IC prior. She is using this framework to ask causal questions about outcomes for galaxies. The long-term goal is to understand the ICs from with the Local Group (and other groups) formed.

Besla (Arizona) and Olsen (NOAO) spoke about the LMC and SMC, with Besla concentrating on arguments about infall and orbit, and Olsen on arguments about stellar and gas dynamics and the Magellanic stream. Besla's talk was full of rich astrophysical (rather than strictly statistical) arguments, and Olsen brought an actual large data set, which was delivered to every participant by USB key!

Kravstov (Chicago) gave a beautiful talk about the timing argument for dark matter, and its update via importance sampling of simulation-based prior samples of local-group analogs. The timing argument is incredibly powerful and surprisingly accurate, he finds. This, combined with the Besla arguments, suggested a whole host of new projects to be doing with simulations and data.

There were so many great talks today, I can't even think to mention them all, but I should shout out VanderPlas (UW), who told us not to be afraid about having more parameters than data, and also is finding RR Lyrae stars automatically with clever generalizations of the periodogram. Foreman-Mackey told the crowd not to do 1/Vmax weighting of incomplete data (and what they should do instead). Farr (Chicago) blew us away with a new, very simple ensemble sampler, that could be merged beautifully with emcee. Martin (Strasbourg) took us to school with his mixture-modeling of the entire halo of M31, and Lang did more of similar for the PHAT project's (overwhelmingly large) astrometric solution. Martin made a nice point about the comparison of M31 with simulations: The data show a lot of substructure relative to realistic stellar halo simulations for galaxies at similar mass. This led to some very useful discussion.