Helium reionization in the presence of self-annihilating clumpy dark matter

Abstract

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 α forest may help to assess whether the evolution is consistent with the contribution from quasars alone, or if an additional component may be required.

Impact of Sommerfeld enhancement on helium reionization via WIMP dark matter

“…Due to this early saturation, the clumping factor of the self-annihilating dark matter turns out to be the same as the case without Sommerfeld Enhancement. As in our previous study [8], the contribution of dark matter annihilation to helium reionization becomes most relevant between redshifts of 2 − 5, where the contribution from quasars also rises significantly. Also in the presence of a Sommerfeld enhancement, it is thus still difficult to disentangle both contributions, requiring a rather precise census of energetic photons produced via AGN.…”

“…As recently shown by [8], the same may occur during the epoch of helium reionization, where highly energetic particles produced in dark matter annihilation events contribute to the transition from the singly-ionized state to the doubly-ionized state of helium. As shown in that study, the main contribution to this effect arises around redshifts of 2 − 4, where the clumpiness in the dark matter distribution is strongly increasing, thus considerably enhancing the annihilation signal.…”

“…In this investigation, we build up on the framework previously developed by [8], which will be extended here to include the effect of Sommerfeld enhancement. In the following, we summarize the main equations with a brief discussion of the main physical processes.…”

“…This approach going back to [30] assumes that the dark matter particles annihilate into high-energy photons, which allows to determine the energy fractions going into heating, ionization and excitation. While [30] employed the simplified approach by [59], we generalized the latter approach to include also the double-ionization of helium [8] adopting the formalism of [22]. Direct annihilation of dark matter particles to photons with thermal cross-sections however violate the HESS results [20].…”

“…We present the overall framework of our investigation in section II, providing a summary of the previous formalism outlined by [8]. The specific effects due to dark matter annihilation, including our treatment of clumping and Sommerfeld enhancement, are then described in section III.…”