Ghostly Galaxies as Solitons of Bose-Einstein Dark Matter
  The large dark cores of common dwarf galaxies are unexplained by the standard
heavy particle interpretation of dark matter.
  This puzzle is exacerbated by the discovery of a very large but barely
visible, dark matter dominated galaxy Antlia II orbiting the Milky Way,
uncovered by tracking star motions with the {\t Gaia} satellite. Although
Antlia II has a low mass, its visible radius is more than double any known
dwarf galaxy, with an unprecedentedly low density core. We show that Antlia II
favors dark matter as a Bose-Einstein condensate, for which the ground state is
a stable soliton with a core radius given by the de Broglie wavelength. The
lower the galaxy mass, the larger the de Broglie wavelength, so the least
massive galaxies should have the widest soliton cores of lowest density. An
ultra-light boson of $m_\psi \sim 1.1 \times10^{-22}$ eV, accounts well for the
large size and slowly moving stars within Antlia II, and agrees with boson mass
estimates derived from the denser cores of more massive dwarf galaxies. For
this very light boson, Antlia II is close to the lower limiting Jeans scale for
galaxy formation permitted by the Uncertainty Principle, so other examples are
expected but none significantly larger in size. This simple explanation for the
puzzling dark cores of dwarf galaxies implies dark matter as an ultra-light
boson, such as an axion generic in String Theory.
