Constraints on MACHO Dark Matter from Compact Stellar Systems in
  Ultra-Faint Dwarf Galaxies
  I show that a recently discovered star cluster near the center of the
ultra-faint dwarf galaxy Eridanus II provides strong constraints on massive
compact halo objects (MACHOs) of >~5 M_sun as the main component of dark
matter. MACHO dark matter will dynamically heat the cluster, driving it to
larger sizes and higher velocity dispersions until it dissolves into its host
galaxy. The stars in compact ultra-faint dwarf galaxies themselves will be
subject to the same dynamical heating; the survival of at least ten such
galaxies places independent limits on MACHO dark matter of masses >~10 M_sun.
Both Eri II's cluster and the compact ultra-faint dwarfs are characterized by
stellar masses of just a few thousand M_sun and half-light radii of 13 pc (for
the cluster) and ~30 pc (for the ultra-faint dwarfs). These systems close the
~20--100 M_sun window of allowed MACHO dark matter and combine with existing
constraints from microlensing, wide binaries, and disk kinematics to rule out
dark matter composed entirely of MACHOs from ~10$^{-7}$ M_sun up to arbitrarily
high masses.
