Do we need stars to reionize the universe at high redshifts? Early reionization by decaying heavy sterile neutrinos

A remarkable result of the Wilkinson Microwave Anisotropy Probe (WMAP) observations is that the universe was significantly reionized at large redshifts. The standard explanation is that massive stars formed early and reionized the universe around redshift z approximate to 17. Here we explore an alternative possibility in which the universe was reionized in two steps. An early boost of reionization is provided by a decaying sterile neutrino whose decay products, relativistic electrons, result in partial ionization of the smooth gas. We demonstrate that a neutrino with a mass of m(nu) similar to 200 MeV and a decay time of t similar to 4 x 10(15) s can account for the electron scattering optical depth tau approximate to 0.16 measured by WMAP without violating existing astrophysical limits on the cosmic microwave and gamma-ray backgrounds. Reionization is then completed by subsequent star formation at lower redshifts. This scenario alleviates constraints on structure formation models with reduced small-scale power, such as those with a running or tilted scalar index, or warm dark matter models.