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 focus on two IMBH formation scenarios; our conservative scenario where IMBHs are remnants of Population-III stars, and our optimistic scenario where IMBHs are formed in protogalactic disks. In both scenarios, their formation in pregalactic halos at high redshift lead to the formation of minispikes that are bright sources of gamma-ray photons. Taking into account minispike depletion processes, we only sum contributions from a cosmological distribution of IMBHs with maintained minispikes. Our conservative scenario (BH mass 10(2)M with a r(-3/2) minispike) predicts gamma-ray fluxes that are an order larger than the equivalent flux, using the same DM parameters (mass 100 GeV and annihilation cross section 3x10(-26) cm(3) s(-1)), from the host halo without IMBH minispikes. Our optimistic scenario (BH mass 10(5)M with a r(-7/3) minispike) predicts fluxes that are three orders larger, that can reach current EGB observations taken by EGRET (DM parameters as above). This fact may serve interesting consequences for constraining DM parameters and elucidating the true nature of IMBHs. Additionally, we determine the spectra of DM annihilation into monochromatic gamma rays, and show that its flux can be within observational range of GLAST, providing a potential "smoking-gun" signature of DM.