The Dark Matter Annihilation Signal from Galactic Substructure: Predictions for GLAST

We present quantitative predictions for the detectability of individual Galactic dark matter subhalos in gamma-rays from dark matter pair annihilations in their centers. Our method is based on a hybrid approach, employing the highest resolution numerical simulations available (including the recently completed one billion particle Via Lactea II simulation) as well as analytical models for the extrapolation beyond the simulations' resolution limit. We include a self-consistent treatment of subhalo boost factors, motivated by our numerical results, and a realistic treatment of the expected backgrounds that individual subhalos must outshine. We show that for reasonable values of the dark matter particle physics parameters (M_X ~ 50 - 500 GeV and <sigma*v> ~ 10^-26 - 10^-25 cm^3/s) GLAST may very well discover a few, even up to several dozen, such subhalos, at 5 sigma significance, and some at more than 20 sigma. We predict that the majority of luminous sources would be resolved with GLAST's expected angular resolution. For most observer locations the angular distribution of detectable subhalos is consistent with a uniform distribution across the sky. The brightest subhalos tend to be massive (median Vmax of 24 km/s) and therefore likely hosts of dwarf galaxies, but many subhalos with Vmax as low as 5 km/s are also visible. Typically detectable subhalos are 20 - 40 kpc from the observer, and only a small fraction are closer than 10 kpc. The total number of observable subhalos has not yet converged in our simulations, and we estimate that we may be missing up to 3/4 of all detectable subhalos.