Primordial black holes with mass 10¹⁶-10¹⁷ g and reionization of the Universe

Primordial black holes (PBHs) with mass $10^{16}-10^{17}$ g almost escape constraints from observations so could essentially contribute to dark matter density. Hawking evaporation of such PBHs produces with a steady rate $\gamma$- and $e^{\pm}$-radiations in MeV energy range, which can be absorbed by ordinary matter. Simplified estimates show that a small fraction of evaporated energy had to be absorbed by baryonic matter what can turn out to be enough to heat the matter so it is fully ionized at the redshift $z\sim 5... 10$. The result is found to be close to a borderline case where the effect appears, what makes it sensitive to the approximation used. In our approximation, degree of gas ionization reaches 50-100% by $z\sim 5$ for PBH mass $(3...7)\times 10^{16}$ g with their abundance corresponding to the upper limit.