The fuzzy dark matter ( FDM ) model treats DM as a bosonic field with astrophysically large de Broglie wavelength .
A striking feature of this model is \mathcal { O } ( 1 ) fluctuations in the dark matter density on time scales which are shorter than the gravitational timescale .
Including for the first time the effect of core oscillations , we demonstrate how such fluctuations lead to heating of star clusters , and thus an increase in their size over time .
From the survival of the old star cluster in Eridanus II we infer m _ { a } \gtrsim 0.6 \rightarrow 1 \times 10 ^ { -19 } \text { eV } within modelling uncertainty if FDM is to compose all of the DM , and derive constraints on the FDM fraction at lower masses .
The subhalo mass function in the Milky Way implies m _ { a } \gtrsim 0.8 \times 10 ^ { -21 } \text { eV } to successfully form Eridanus II .
The region between 10 ^ { -21 } \text { eV } and 10 ^ { -20 } \text { eV } is affected by narrow band resonances .
However , the limited applicability of the diffusion approximation means that some of this region may still be consistent with observations of Eridanus II .