Dark matter annihilation from intermediate-mass black holes: Contribution to the extragalactic gamma-ray background

We investigate contributions to the extragalactic gamma-ray background (EGB) due to neutralino dark matter (DM) pair-annihilation into photons, from DM density enhancements (minispikes) surrounding intermediate-mass black holes (IMBHs). We take into account the effect of mergers, which works to destroy minispikes, and sum only the gamma-ray flux from a cosmological distribution of IMBHs with maintained minispikes. For our most conservative case, considering IMBH of mass $10^2 M_\odot$ and minispike of gradient $r^{-3/2}$, we find that the predicted flux is one order larger than the equivalent flux, using the same DM parameters, from the host halo alone (i.e. without IMBH minispikes). For our most optimistic case, considering massive $10^5 M_\odot$ IMBHs with $r^{-7/3}$ minispikes, the predicted flux is three orders larger and can reach current EGB observations taken by EGRET (for DM particle parameters mass $100 \mathrm{GeV}$ and cross section $3\times10^{-26} \mathrm{cm^3 s^{-1}}$). This fact has interesting implications for constraining DM parameters and elucidating the true nature of intermediate and supermassive black holes. Regarding revealing DM parameters, we also determine the characteristic spectral shape of DM annihilation into monochromatic gamma rays, and show that its flux is within possible observational range of GLAST. Additionally, we note that contributions to the EGB from minispikes are weakly dependent on DM parameters, so that in the most optimistic case, GLAST may be able to probe cross sections down to $\sigma\mathit{v}\simeq10^{-29} \mathrm{cm^3 s^{-1}}$