  The recently discovered stellar system Ursa Major III/UNIONS 1 (UMa3/U1) is
the faintest known Milky Way satellite to date. With a stellar mass of
$16^{+6}_{-5}\,\rm M_\odot$ and a half-light radius of $3\pm1$pc, it is either
the darkest galaxy ever discovered or the faintest self-gravitating star
cluster known to orbit the Galaxy. Its line-of-sight velocity dispersion
suggests the presence of dark matter, although current measurements are
inconclusive because of the unknown contribution to the dispersion of potential
binary stars. We use $N$-body simulations to show that, if self-gravitating,
the system could not survive in the Milky Way tidal field for much longer than
a single orbit (roughly 0.4Gyr), which strongly suggests that the system is
stabilized by the presence of large amounts of dark matter. If UMa3/U1 formed
at the center of a ~$10^9\rm M_\odot$ cuspy LCDM halo, its velocity dispersion
would be predicted to be of order ~1km/s. This is roughly consistent with the
current estimate, which, neglecting binaries, places $\sigma_{\rm los}$ in the
range 1 to 4km/s. Because of its dense cusp, such a halo should be able to
survive the Milky Way tidal field, keeping UMa3/U1 relatively unscathed until
the present time. This implies that UMa3/U1 is plausibly the faintest and
densest dwarf galaxy satellite of the Milky Way, with important implications
for alternative dark matter models and for the minimum halo mass threshold for
luminous galaxy formation in the LCDM cosmology. Our results call for
multi-epoch high-resolution spectroscopic follow-up to confirm the dark matter
content of this extraordinary system.
