Impact of Primordial Ultracompact Minihaloes on the Intergalactic Medium
  and First Structure Formation
  (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.
