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Dark matter annihilation in galactic substructure would imprintcharacteristic angular signatures on the all-sky map of the diffuse gamma-raybackground. We study the gamma-ray background anisotropy due to the subhalosand discuss detectability at Fermi Gamma-ray Space Telescope. We deriveanalytic formulae that enable to directly compute the angular power spectrum,given parameters of subhalos. As our fiducial subhalo models, we adopt M^{-1.9}mass spectrum, subhalos radial distribution suppressed towards the galacticcenter, and luminosity profile of each subhalo dominated by its smoothcomponent. We find that, for multipole regime corresponding to \theta <~ 5 deg,the angular power spectrum is dominated by a noise-like term, with suppressiondue to internal structure of relevant subhalos. If the mass spectrum extendsdown to Earth-mass scale, then the subhalos would be detected in the anisotropywith Fermi at angular scales of ~10 deg, if their contribution to the gamma-raybackground is larger than ~20%. If the minimum mass is around 10^4 M_{sun}, onthe other hand, the relevant angular scale for detection is ~1 deg, and theanisotropy detection requires that the subhalo contribution to the gamma-raybackground intensity is only ~4%. These can be achieved with a modest boost forparticle physics parameters. We also find that the anisotropy analysis could bea more sensitive probe for the subhalos than individual detection. We alsostudy dependence on model parameters, where we reach the similar conclusionsfor all the models investigated. The analytic approach should be very usefulwhen Fermi data are analyzed and the obtained angular power spectrum isinterpreted in terms of subhalo models.

Gamma-ray background anisotropy from galactic dark matter substructure