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

Abstract

(Abridged) Ultracompact Minihaloes (UCMHs), which formed by dark matter accretion onto primordial black holes (PBHs) or initial dark matter overdensity produced by the primordial density perturbation, provide a new type of compact dark matter structure to ionize and heat the IGM after matter-radiation equality z_eq, which is much earlier than the formation of the first cosmological dark halo structure and later first stars. We show that dark matter annihilation density contributed by UCMHs can totally dominated over the homogenous dark matter annihilation background even for a tiny UCMH abundance, and provide a new gamma-ray background in the early Universe. The IGM ionization fraction x_ion and gas temperature T_m can be increased from the recombination residual and adiabatically cooling in the absence of energy injection to the highest value of x_ ion 0.1 and T_m ~ 5000 K at z>10 for the upper bound UCMH abundance constrained by the CMB optical depth. A small fraction of UCMHs are seeded by PBHs. The X-ray emission from gas accretion onto PBHs may totally dominated over dark matter annihilation and become the main cosmic ionization source, but the constraints of gas accretion rate and X-ray absorption by the baryon accumulation within the UCMHs and accretion feedback show that X-ray emission can only be a promising source much later than UCMH annihilation at z<z_m<1000, where z_m depends on the masses of PBHs, their host UCMHs, and the dark matter particles. Also, UCMH radiation including both annihilation and X-ray emission can significantly suppress the low mass first baryonic structure formation. The effects of UCMHs radiation on the baryonic structure evolution are quite small for the gas temperature after virialization, but more significant to enhance the gas chemical quantities such as the ionization fraction and molecular hydrogen abundance in the baryonic objects.