As my loyal readers both know, in 2005 Willman discovered the least luminous galaxy in the Universe: it is a tiny companion to the Milky Way, discovered initially as an over-density of about eight red-giant stars in a small patch of the sky in Ursa Major. Willman's objective, statistical search for such objects has put very strong limits on what else can be out there. Blanton, Willman, and I discussed some of the technical details of her forthcoming paper on these limits. This is a difficult project, but very important, given that dark-matter models predict enormous numbers of companions in the dark sector, and galaxy-formation models have very different mechanisms for explaining which are observable as galaxies.
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