Background radiation from sterile neutrino decay and reionization

Sterile neutrinos are one of the most promising Warm Dark Matter candidates. By considering their radiativeand pion-decay channels, we derive the allowed contribution of sterile neutrinos to the X-ray, optical and near-infrared cosmic backgrounds. The X-ray background puts a strong constraint on the mass of radiatively decaying neutrinos (mνs < ∼ 14 keV), whereas the allowed mass range for pion-decay neutrinos (for a particle lifetime > 4 × 10 s) is 150 ≤ mνs/MeV ≤ 500. Taking into account these constraints, we find that sterile neutrinos do not significantly contribute to the optical and near-infrared background. We further consider the impact of sterile neutrinos on reionization. We find that the Thomson optical depth due to sterile neutrinos is τe = (0.4 − 3) × 10 −2 in the case of radiative decays, and it is ≈ 10 for the pion-decay channel. We conclude that these particles must have played only a minor role in cosmic reionization history.