Observability of gamma rays from neutralino annihilations in the Milky Way substructure

We estimate the probability of detecting gamma rays from the annihilation of neutralino dark matter in dense, central regions of Milky Way substructure. We characterize galactic substructure statistically based on Monte Carlo realizations of the formation of a Milky Way–like halo using a semianalytic method that has been calibrated against the results of high-resolution N-body simulations. We find that it may be possible for the upcoming experiments GLAST and VERITAS, working in concert, to detect gamma rays from dark matter substructure if the neutralino is relatively light ${(M}_{\chi }\lesssim 100\mathrm{GeV}),$ while for $M_{\chi }\gtrsim 500\mathrm{GeV}$ such a detection would be unlikely. We perform most of our calculations within the framework of the standard $\Lambda \mathrm{CDM}$ cosmological model; however, we also investigate the robustness of our results to various assumptions and find that the probability of detection is sensitive to poorly constrained input parameters, particularly those that characterize the primordial power spectrum. Specifically, the best-fitting power spectrum of the WMAP team, with a running spectral index, predicts roughly a factor of fifty fewer detectable subhalos compared to the standard $\Lambda \mathrm{CDM}$ cosmological model with scale-invariant power spectrum. We conclude that the lack of a detected gamma-ray signal gives very little information about the supersymmetric parameter space due to uncertainties associated with both the properties of substructure and cosmological parameters.