Dwarf Galaxies and the Nature of Dark Matter

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The past few decades have seen the emergence of a paradigm for the content and geometry of the Universe. This paradigm (usually referred to by its acronym LCDM) envisions the matter-energy content of the Universe as dominated by the evolving balance between radiation and two exotic components: cold dark matter and dark energy. However improbable this may sound, LCDM successfully accounts for most cosmological observations on large scales, and is widely regarded as the first successful cosmological theory ever devised. Its success on large scales notwithstanding, LCDM makes predictions on small scales that apparently run into conflict with observations of faint galaxies. These issues have surfaced as the completion of photometric and redshift surveys of large portions of the sky has revolutionized our understanding of faint galaxies, extending the inventory of dwarf galaxies to luminosities so faint that they blur the traditional distinction between galaxies and star clusters. These observations have highlighted potentially fatal challenges to LCDM on small scales, including the lack of a conspicuous “cusp” of dark matter at the center of dwarf galaxies; the scarcity of low-mass galaxies compared with the huge predicted abundance of low-mass dark matter halos capable, in principle, of hosting them; as well as oddities in the kinematic and spatial arrangement of the dwarf satellites of the Milky Way and Andromeda galaxies. All of these problems are the subject of active investigation, especially since plausible (but radical) modifications to the LCDM paradigm, such as “warm” or “self-interacting” dark matter, provide attractive alternatives. The confrontation of data with LCDM predictions for the smallest galaxies known thus holds the promise to validate or challenge the established paradigm on scales far removed from those used to tune its main parameters. The time seems ripe for a concerted effort designed to assess the cosmological significance of Nature's smallest extragalactic objects. I propose to study this topic using a variety of observational and theoretical tools, from detailed N-body simulation of the formation of dark matter halos, to gas-dynamical simulation of the formation of luminous dwarf galaxies, to direct observational efforts designed to survey the faintest galaxies in our Local Universe and to understand their impact on the formation of our Milky Way galaxy.