Gamma-Ray Constraints on Neutralino Dark Matter Clumps in the Galactic Halo

High resolution simulations show that galaxy and cluster halos are formed by the constant merging of many smaller halos. The presence of smaller mass halos in our Galactic halo overwhelms the contribution to the gamma-ray background from the smooth halo component. EGRET limits this gamma-ray flux and places severe constraints on the neutralino parameters. We calculate the spectrum and angular distribution in the sky of the gamma-ray flux due to neutralino annihilation in the clumpy halo of the Galaxy. If we model clumps as SIS, the flux of gamma-rays for neutralinos with masses between 50 GeV and a few TeV and $\sigma v \ga 2 \times 10^{-35} (m_{\chi}/{\rm 100 GeV})^{2}$ cm$^3$/s are ruled out by EGRET. If clumps are best described by the Moore et al. profile, neutralinos less massive than a few TeV and $ \sigma v \ga 2.5 \times 10^{-28}(m_{\chi}/{\rm 100 GeV})^{1.5}$ cm$^3$/s are also ruled out by EGRET. The less concentrated NFW clumps are constrained to have $ \sigma v \la 1.2 \times 10^{-26} (m_{\chi}/{\rm 100 GeV})^{1.5}$ cm$^3$/s for $m_{\chi} \la 3$ TeV and can be further constrained by future GLAST measurements of the diffuse $\gamma$-ray background. In addition, GLAST will easily detect the angular anisotropy from a clumpy halo as it varies by orders of magnitude from the Galactic center to the anti-center. The $\gamma$-ray emission from the clumpy Galactic halo already places severe constraints either on the neutralino parameters or on the dark matter density structure. The clumpy neutralino halo is an ideal target for future $\gamma$-ray experiments such as GLAST.