Abstract
Recent discoveries of large halos of stars and dark matter around some of the lowest mass galaxies defy expectations that dwarf galaxies should be small and dense. Here we find large halos are a general feature of the well known dwarfs orbiting the Milky Way and also for the isolated dwarfs in the Local Group. Furthermore, these halos are seen to surround a dense core within each dwarf, with a clear density transition visible between the core and the halo at a radius of $\ensuremath{\simeq}1.0\text{ }\text{ }\mathrm{kpc}$. This common core-halo structure is hard to understand for standard heavy particle dark matter where featureless, concentrated profiles are predicted, whereas dark matter as a Bose-Einstein condensate, $\ensuremath{\psi}\mathrm{DM}$, naturally accounts for the observed profiles, predicting a dense soliton core in every galaxy surrounded by a tenuous halo of interfering waves. We show that the stellar profiles are accurately fitted by the core-halo structure of $\ensuremath{\psi}\mathrm{DM}$, with only one fixed parameter, the boson mass. We also find independent consistency with the stellar velocity dispersions measured in these dwarf galaxies, which peak at the core radius and fall in the halo, at a level consistent with a boson mass of $\ensuremath{\simeq}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$, based on independent dynamical work. Hence, dark matter comprised of light bosons, such as the axions generic in string theory, provides a compelling solution for the structure of dwarf galaxies with stars that simply trace the dark matter profile of a Bose-Einstein condensate.