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 largeredshifts. The standard explanation is that massive stars formed early andreionized the universe around redshift z=17. Here we explore an alternativepossibility, in which the universe was reionized in two steps. An early boostof reionization is provided by a decaying sterile neutrino, whose decayproducts, relativistic electrons, result in partial ionization of the smoothgas. We demonstrate that a neutrino with a mass of m_nu ~ 200 MeV and a decaytime of t ~ 4 * 10^{15} s can account for the electron scattering optical depthtau=0.16 measured by WMAP without violating existing astrophysical limits onthe cosmic microwave and gamma ray backgrounds. Reionization is then completedby subsequent star formation at lower redshifts. This scenario alleviatesconstraints on structure formation models with reduced small-scale power, suchas those with a running or tilted scalar index, or warm dark matter models.Comment: Improved the discussion of electron-CMB interaction; describe the reionization process in more detail; conclusions unchanged; few references added (as accepted by ApJ