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 greater than or similar to GeV, are v(k), the recoil speed of the daughter particle, and tau, 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 x 10(6)M(circle dot), although constraints based on M-300 in the classical dwarfs and the overall velocity function are competitive for v(k) greater than or similar to 50 km s(-1). In general, we find that tau less than or similar to 30 Gyr is ruled out for 20 km s(-1) less than or similar to v(k) less than or similar to 200 km s(-1), although we find that the limits on tau for fixed v(k) can change by a factor of similar to 3 depending on the star-formation histories of the satellites. We advocate using the distribution of M-300 in Milky Way satellites, determined by next-generation all-sky surveys and follow-up spectroscopy, as a probe of dark-matter physics.