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Individual extragalactic dark matter halos, such as those associated withnearby galaxies and galaxy clusters, are promising targets for searches forgamma-rays from dark matter annihilation. We review the predictions for theannihilation flux from individual halos, focusing on the effect of currentuncertainties in the concentration-mass relation and the contribution from halosubstructure, and also estimating the intrinsic halo-to-halo scatter expected.After careful consideration of recent simulation results, we conclude that theconcentrations of the smallest halos, while well determined at high redshift,are still uncertain by a factor of 4-6 when extrapolated to low redshift. Thisin turn produces up to two orders of magnitude uncertainty in the predictedannihilation flux for any halo mass above this scale. Substructure evolution,the small-scale cutoff to the power spectrum, cosmology, and baryonic effectsall introduce smaller, though cumulative, uncertainties. We then considerintrinsic variations from halo to halo. These arise from variations inconcentration and substructure, leading to a scatter of $\sim$ 2.5 in thepredicted annihilation luminosity. Finally, we consider the problem ofdetecting gamma-rays from annihilation, given the expected contributions fromother sources. We estimate the signal-to-noise ratio for gamma-ray detection asa function of halo mass, assuming that cosmic rays from star formation are themain noise source in the detection. This calculation suggests that group-scalehalos, individually or in stacks, may be a particularly interesting target forthe next generation of annihilation searches.

Searching for dark matter annihilation from individual halos: uncertainties, scatter and signal-to-noise ratios