Constraining self-interacting dark matter with the Milky Way's dwarf spheroidals

Self-interacting dark matter is an attractive alternative to the cold dark matter paradigm only if it is able to substantially reduce the central densities of dwarf-size haloes while keeping the densities and shapes of cluster-size haloes within current constraints. Given the seemingly stringent nature of the latter, it was thought for nearly a decade that self-interacting dark matter would be viable only if the cross-section for self-scattering was strongly velocity dependent. However, it has recently been suggested that a constant cross-section per unit mass of sigma(T)/m similar to 0.1 cm(2) g(-1) is sufficient to accomplish the desired effect. We explicitly investigate this claim using high-resolution cosmological simulations of a Milky Way-size halo and find that, similarly to the cold dark matter case, such cross-section produces a population of massive subhaloes that is inconsistent with the kinematics of the classical dwarf spheroidals, in particular with the inferred slopes of the mass profiles of Fornax and Sculptor. This problem is resolved if sigma(T)/m similar to 0.1 cm(2) g(-1) at the dwarf spheroidal scales. Since this value is likely inconsistent with the halo shapes of several clusters, our results leave only a small window open for a velocity-independent self-interacting dark matter model to work as a distinct alternative to cold dark matter.