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 \gtrsim 5 M _ { \odot } 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 \gtrsim 10 M _ { \odot } .
Both Eri II ’ s cluster and the compact ultra-faint dwarfs are characterized by stellar masses of just a few thousand M _ { \odot } and half-light radii of 13 pc ( for the cluster ) and \sim 30 pc ( for the ultra-faint dwarfs ) .
These systems close the \sim 20–100 M _ { \odot } 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 \sim 10 ^ { -7 } M _ { \odot } up to arbitrarily high masses .