Relic keV Sterile Neutrinos and Reionization

A sterile neutrino with a mass of several keV can account for cosmological dark matter, as well as explain the observed velocities of pulsars. We show that x rays produced by the decays of these relic sterile neutrinos can boost the production of molecular hydrogen, which can speed up the cooling of gas and the early star formation, which can, in turn, lead to a reionization of the Universe at a high enough redshift to be consistent with the Wilkinson Microwave Anisotropy Probe results.

Figure 1

The rate $k_m$ of molecular hydrogen production as a function of redshift. For illustrative purposes, the gas temperature is assumed to be close to that of the CMBR (which is a good assumption, at least, for $z>150$).

Figure 2

The fraction of ions, $x_e$, in the absence of sterile neutrinos (thin line), and for the dark-matter sterile neutrinos with masses 4 and 7 keV (lower and upper thick lines, respectively). Also shown is the limit of baryon decoupling from CMBR (dashed line). Below the dashed line, gas cooling is unaffected by the CMBR.