  We consider a class of late-decaying dark-matter models, in which a dark
matter particle decays to a heavy stable daughter of approximately the same
mass, together with one or more relativistic particles which carry away only a
small fraction of the parent rest mass. Such decays can affect galactic halo
structure and evolution, and have been invoked as a remedy to some of the small
scale structure-formation problems of cold dark matter. There are existing
stringent limits on the dark matter lifetime if the decays produce photons. By
considering examples in which the relativistic decay products instead consist
of neutrinos or electron-position pairs, we derive stringent limits on these
scenarios for a wide range of dark matter masses. We thus eliminate a sizable
portion of the parameter space for these late decay models if the dominant
decay channel involves Standard Model final states.
