Marla Geha (Yale) made a surprise visit to Flatiron today, and bombed the weekly Stars and Exoplanets Meeting with a discussion of the challenges of measuring velocity dispersions (and hence masses, and hence dark-matter-annihilation limits) in ultra-faint dwarf galaxies in the halo of the Milky Way. As my loyal reader knows, this problem is very similar to problems we are working on at Flatiron around extreme-precision radial-velocity (EPRV) spectroscopy. Geha's problem is both harder and easier. It is easier because she only needs km/s (not cm/s) precision. It is harder because she has to use a slit spectrograph and point it at very faint stars! It is easier because she has both sky emission lines and telluric absorption lines to help calibrate. It is harder because differences in slit illumination mean that the sky lines and the telluric absorption don't agree for the wavelength calibration!
After stars meeting, the conversation continued among Geha, Bedell (Flatiron), and me. We discussed many things, including the point that the offset between tellurics and sky lines is a wavelength offset, not a radial-velocity offset. Or it is even something more sophisticated, related to the spectrograph optics. We discussed the point that her problems are fundamentally hierarchical, because some parameters are associated with a star, some with an exposure, some with a slit-mask, some with a time and so on. We also discussed how the wobbble framework that Bedell and I have built could be extended to capture these effects. It's certainly possible. Oh I nearly forgot: We also discussed masking and apodization of sky lines and telluric lines in the science spectra, and how to do that without biasing down-stream measurements. Spergel (Flatiron) pointed us to some literature that he was pleased to say is older than any of us (Spergel included).
I should say that Geha's admirable goal is to re-reduce all of the nearly 105df stellar spectra in the DEIMOS archive! Now that's my kind of project.
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