PHAT Camp, day one

A week-long sprint on the Panchromatic HST Andromeda Treasury project started today, with Dan Weisz, Morgan Fouesneau, Cliff Johnson, Karl Gordon, and of course PI Dalcanton in Heidelberg, along with semi-locals me, Rix, and Foreman-Mackey. Actually, you have to count Dalcanton as a quasi-local too.) We started by laying down the law: No-one leaves Heidelberg without the figures for a paper on the present-day mass function in young stellar clusters. We then proceeded to work through the status and conceptual issues, with reviews of work by Gordon on likelihood functions (think converting photometry to mass estimates) and Weisz on hierarchical methods for getting the PDMF out of noisy and incomplete data. A fight started to break out about completeness functions; we are going to have to resolve this tomorrow. The key thing that we at PHAT Camp will not do is upweight incomplete data by the inverse of the completeness; that absurdity weights arbitrarily highly the least likely data points! But we still don't agree on how the completeness is best included in the fits. This should be a matter of probability theory—not taste—but we are still a bit confused (myself included). We all started writing code for the various parts of the project.

In a side discussion, Dalcanton, Rix, Johnson, and I discussed evolutionary models for fading, dissolving, evaporating, and expanding stellar clusters, bound and unbound. Johnson began the discussion with a very nice review of the current literature and the controversies therein. We came up with a continuity equation formalism and assigned Johnson the task of making a toy model. My student Jiang is doing similar things with galaxy evolution back in NYC.