Abstract

The reionization of helium describes the transition from its singly ionized state to a doubly ionized state in the intergalactic medium. This process is important for the thermal evolution of the intergalactic medium and influences the mean free path of photons with energies above 54.4 eV. While it is well known that helium reionization is mostly driven by the contribution of energetic quasars at z $\lt$6 , we study here how helium reionization proceeds if there is an additional contribution due to the annihilation of dark matter. We explore the effects of different dark matter profiles for the dark matter clumping factor, which can significantly enhance the annihilation rate at late times. We find that the presence of dark matter annihilation enhances the He$^{++}$ abundance at early stages where it would be zero within the standard model, and it can further increase during structure formation, reflecting the increase of the dark matter clumping factor. The latter is, however, degenerate with the buildup of the quasar contribution, and we therefore expect no significant changes at late times. We expect that future studies of the He$^{+}$ Lyman {$\alpha$} forest may help to assess whether the evolution is consistent with the contribution from quasars alone, or if an additional component may be required.