Background radiation from sterile neutrino decay and reionization

Sterile neutrinos are one of the most promising warm dark matter (WDM) candidates. By considering their radiative- and pion-decay channels, we derive the allowed contribution of sterile neutrinos to the X-ray, optical and near-infrared (NIR) cosmic backgrounds. The X-ray background puts a strong constraint on the mass of radiatively decaying neutrinos (m(nu s)less than or similar to 14 keV), whereas the allowed mass range for pion-decay neutrinos (for a particle lifetime > 4 x 10(17) s) is 150 <= m(nu s)/MeV <= 500. Taking into account these constraints, we find that sterile neutrinos do not significantly contribute to the optical and NIR background. We further consider the impact of sterile neutrinos on reionization. We find that the Thomson optical depth due to sterile neutrinos is tau(e)= (0.4-3) x 10(-2) in the case of radiative decays and it is approximate to 10(-3) for the pion-decay channel. We conclude that these particles must have played only a minor role in cosmic reionization history.