The Darkest Hour Before Dawn: Contributions to Cosmic Reionization from Dark Matter Annihilation and Decay

摘要

Dark matter annihilation or decay could have a significant impact on theionisation and thermal history of the universe. In this paper, we study thepotential contribution of dark matter annihilation (s-wave- orp-wave-dominated) or decay to cosmic reionisation, via the production ofelectrons, positrons and photons. We map out the possible perturbations to theionisation and thermal histories of the universe due to dark matter processes,over a broad range of velocity-averaged annihilation cross-sections/decaylifetimes and dark matter masses. We have employed recent numerical studies ofthe efficiency with which annihilation/decay products induce heating andionization in the intergalactic medium, and in this work extended them down toa redshift of $1+z = 4$ for two different reionisation scenarios. We alsoimprove on earlier studies by using the results of detailed structure formationmodels of dark matter haloes and subhaloes that are consistent with up-to-date$N$-body simulations, with estimates on the uncertainties that originate fromthe smallest scales. We find that for dark matter models that are consistentwith experimental constraints, a contribution of more than 10% to theionisation fraction at reionisation is disallowed for all annihilationscenarios. Such a contribution is possible only for decays intoelectron/positron pairs, for light dark matter with mass $m_\chi \lesssim $ 100MeV, and a decay lifetime $\tau_\chi \sim 10^{24} - 10^{25}$ s.