We perform a set of cosmological simulations of structure formation in a mixed dark matter ( MDM ) model .
Our model is motivated by the recently identified 3.5 { keV } X-ray line that can be explained by the decay of non-resonantly produced sterile neutrinos , if they account for 10 \text { - - } 60 \% of the dark matter in the Universe .
The non-resonantly produced sterile neutrino has sizable free-streaming length and hence behaves effectively as warm dark matter ( WDM ) .
Assuming the rest of dark matter is composed of some stable and cold particles , i.e .
cold dark matter ( CDM ) , we follow the coevolution of the CDM and WDM density perturbations .
Specifically , we consider the models with the warm component fraction of r _ { warm } = 0.25 and 0.5 .
Our MDM model predicts that the comoving Jeans length at the matter-radiation equality is close to that of the thermally produced warm dark matter model with particle mass m _ { WDM } = 2.4 { keV } but that the suppression in the fluctuation power spectrum is weaker .
We perform large N -body simulations to study the structure of nonlinear dark halos in the MDM model .
The abundance of substructure is significantly reduced in the MDM model , and hence the so-called small scale crisis is mitigated .
The cumulative maximum circular velocity function ( CVF ) of at least one halo in the MDM models is in good agreement with the CVFs of the observed satellites in the Milky Way and Andromeda .
We argue that the MDM models open an interesting possibility to reconcile the reported 3.5 { keV } line and the internal structure of galaxies .