Decaying Dark Matter and the Deficit of Dwarf Halos

The hierarchical clustering inherent in Lambda-Cold Dark Matter cosmology (Lambda CDM) seems to produce many of the observed characteristics of large-scale structure. But some glaring problems still remain, including the over-prediction (by a factor 10) of the number of dwarf galaxies within the virialized population of the local group. We carry out a detailed calculation of the dwarf halo evolution incorporating the effects of a hypothesized dark-matter decay, D -> D' +l, where D is the unstable particle, D' is the more massive daughter particle and l is the other, lighter (or possibly massless) daughter particle. This process preferentially heats the smaller halos, expanding them during their evolution and reducing their present-day circular velocity. We find that this mechanism can account very well for the factor 4 deficit in the observed number of systems with vel 10-20 km s(-1) compared to those predicted by the numerical simulations, if Delta m/m(D') similar to 5 - 7 x 10(-5) where Delta m is the mass difference between the initial and final states. The corresponding lifetime tau cannot be longer than similar to 30 Gyr, but may be as short as just a few Gyr.