Impact of primordial ultracompact minihaloes on the intergalactic medium and first structure formation

The effects of dark matter annihilation on the evolution of the intergalactic medium (IGM) in the early Universe will be more important if the dark matter structure is more concentrated. Ultracompact minihaloes (UCMHs), which formed through dark matter accretion on to primordial black holes (PBHs) or an initial dark matter overdensity produced by a primordial density perturbation, provide a new type of compact dark matter structure to ionize and heat the IGM after matterradiation equality zeq, which is much earlier than the formation of the first cosmological dark halo structure and later the first stars. We show that the dark matter annihilation density contributed by UCMHs can completely dominate over the homogeneous dark matter annihilation background, even for a tiny UCMH fraction fUCMH=OUCMH(zeq)/ODM= 10-15(1 +z)2(m?c2/100 GeV)-2/3 with a standard thermal-relic dark matter annihilation cross-section, and can provide a new gamma-ray background in the early Universe. UCMH annihilation becomes important to IGM evolution for approximately fUCMH > 10-6(m?c2/100 GeV). The IGM ionization fraction xion and gas temperature Tm can be increased from the recombination residual xion similar to 10-4 and adiabatically cooling Tm? (1 +z)2 in the absence of energy injection, to a maximum value of xion similar to 0.1 and Tm similar to 5000 K at z= 10 for the upper bound on UCMH abundance constrained by the cosmic microwave background optical depth.