Helium reionization in the presence of self-annihilating clumpy dark
  matter
  The reionization of helium describes the transition from its singly ionized
state to a doubly-ionized state in the intergalactic medium (IGM). This process
is important for the thermal evolution of the IGM 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 $z<6$, 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
build-up 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.
