find catastrophes in the stellar distribution

In Zolotov et al (2011) we asked the question: Might tiny dwarf galaxy Willman 1 be just a cusp in the stellar distribution of the Milky Way? If you generically have lines and sheets in phase space—and we very strongly believe that the Milky Way does—then generically you will have folds in those (in non-trivial projections they are required), and those folds generically produce catastrophes (localized regions of very high density) of various kinds (folds, cusps, swallowtails, and so on), which could mimic gravitationally bound or recently disrupted overdensities in the stellar distribution. The cool thing is that the catastrophes have quantitative two-dimensional morphologies that are very strongly constrained by mathematics (not just physics). The likelihood test we did in the Zolotov paper could easily be expanded into a search technique, maybe with some color-magnitude-diagram filtering mixed in. The catastrophes pretty much have to be there so get ready to get rich and famous! If you go there, send email to Scott Tremaine (IAS), who first proposed this idea to me.

Find more of the GD-1 stream

The GD-1 stream spans many tens of degrees in the SDSS data. The stellar density in the stream is inhomogeneous, but the stream appears to be terminated by the survey boundary, and not before. So we should be able to find much more of it! And more stream means better constraints on the mass model for the Milky Way and the formation of cold streams. A few years back we made a model of GD-1, so we can predict where the stream will be on the unobserved parts of the sky and at what heliocentric distance. These properties of the stream set the parameters for a simple (say) three-color ground-based imaging survey to recover the stream in the Southern Hemisphere. Before you go get the observing time, I would recommend looking in the various data archives; there might already be sufficient data out there to map parts of the stream right now.

will chemical tagging work?

Chemical tagging is the name given to the idea that we can match up stars in abundance space (detailed chemical properties) as well as kinematic space to figure out the origin and common orbits of stars in the Milky Way. Because it would be so valuable to figure out that different stars shared a common origin at formation (for things like orbit inference), chemical tagging could enormously improve the precision of any dynamical or galaxy-formation information coming from next-generation surveys.

In the many conversations I have seen about chemical tagging, arguments break out about whether it is possible to measure the chemical abundances of stars of different temperatures and surface gravities comparably. That is: Can we figure out that this F star has the same abundances as this other K star? Or this red giant and this main-sequence star? And it is certainly not clear: Chemical abundances are not measured at enormous precision and there are many possible biases, sources of variance, and systematic error.

My proposal is that we ask these questions not in the space of the outputs of chemical-abundance models but rather in the space of stellar spectroscopy observables. The question becomes not are the models good enough? but rather is there information in the data? And there needs to be information sufficient to distinguish thousands (yes that is the goal) of chemically distinct sub-populations.

If there is sufficient information, then in the dozens-to-hundreds-of-dimensions space of all possible absorption-line measurements (plus stellar temperature), do we see thousands of distinct families of (possibly very complex) one-dimensional loci (each locus being a birth-mass-sequence at fixed chemical abundances and age)? The idea would be to do this purely in the space of spectra but—probably necessarily—relying heavily on models to guide the eye (or really guide the code) where to look.

I have discussed this with Ken Freeman (ANU) and Mike Blanton (NYU), but as far as I know, no-one is working on it. Blanton had the great idea that we don't really need to make spectral features before starting. The question does the distribution of stellar spectra split up into many tiny, thin, curvy lines in spectrum space? can be asked with just well-calibrated spectra. And we have lots of those!