Dark-matter decays and Milky Way satellite galaxies

We consider constraints on a phenomenological dark-matter model consisting of two nearly degenerate particle species using observed properties of the Milky Way satellite galaxy population. The two parameters of this model, assuming the particle masses are ≳ GeV, are v_k, the recoil speed of the daughter particle, and τ, the lifetime of the parent particle. The satellite constraint that spans the widest range of v_k is the number of satellites that have a mass within 300 pc M_(300)>5×10^6M_⊙, although constraints based on M_300 in the classical dwarfs and the overall velocity function are competitive for vk≳50 km s^(-1). In general, we find that τ≲30 Gyr is ruled out for 20 km s^(-1)≲v_k≲200 km s^(-1), although we find that the limits on τ for fixed vk can change by a factor of ~3 depending on the star-formation histories of the satellites. We advocate using the distribution of M300 in Milky Way satellites, determined by next-generation all-sky surveys and follow-up spectroscopy, as a probe of dark-matter physics