CDM structure of dwarf galaxies

An outstanding puzzle in the now-standard dark energy plus cold dark matter (LCDM) cosmological model is how to reproduce observed properties of the Milky Way. Over the past decade, one of the most concerning issues has been the "missing satellites" problem: although LCDM unavoidably predicts that the Milky Way should host a vast population of dark matter satellites, only a dozen or so satellite galaxies had been observed. While the discovery of a new population of ultra-faint dwarf galaxies has been taken as an encouraging sign of reconciliation between theory and observation, I will show that improved understanding of the structure of Milky Way satellites has revealed an additional problem: the majority of the most massive dark matter satellites of the Milky Way predicted in LCDM are too dense to host any of the Milky Way's bright dwarf galaxies. This is a surprising result in the context of galaxy formation theories, which place the bright dwarf satellites of Milky Way in its most massive dark matter subhalos. Potential resolutions may indicate that galaxy formation is markedly different in low-mass dark matter halos than in larger systems, or that the nature of dark matter differs from the LCDM standard of a cold, weakly interacting massive particle.