Simulations of ultralight , \sim 10 ^ { -22 } eV , bosonic dark matter exhibit rich wave-like structure , including a soliton core within a surrounding halo that continuously self-interferes on the de Broglie scale .
We show here that as an inherent consequence , the soliton undergoes a confined random walk at the base of the halo potential .
This is significant for the fate of the ancient central star cluster in Eridanus II , as the agitated soliton gravitationally shakes the star cluster in and out of the soliton on a time scale of \sim 100 Myr , so complete tidal disruption of the star cluster can occur within \sim 1 Gyr .
This destructive effect can be mitigated by tidal stripping of the halo of Eridanus II , thereby reducing the agitation , depending on its orbit around the Milky Way .
Our simulations show the Milky Way tide affects the halo much more than the soliton , so the star cluster in Eridanus II can survive for over 5 Gyr within the soliton if it formed after significant halo stripping .