RADIATIVELY DECAYING NEUTRINOS AND PHOTOIONIZATION OF THE UNIVERSE ATHIGH REDSHIFTS

The ionization history of the universe in the presence of radiatively decaying neutrinos with masses between 30 eV and a few keV is studied. We consider a model in which most of the neutrinos decay into invisib le particles at a rate t(v)(-1) much greater than t(0)(-1), with t(0) being the present age of the universe, while a small fraction of neutr inos, B, decay radiatively. We follow the evolution of the fraction of neutral hydrogen (H I), neutral helium (He I), and singly ionized hel ium (He II) in the diffuse intergalactic medium (IGM), taking into acc ount the absorption of decay photons by hydrogen and helium In the dif fuse IGM and the Ly alpha systems. The constraints on radiatively deca ying neutrinos from the spectrum of cosmic background radiation, SN 19 87A, the cooling of red giants, and the diffuse extragalactic backgrou nd of photons are also considered. We derive the parameter space-spann ed by the mass of the unstable neutrino m(v), t(v), and B-allowed by t he Gunn-Peterson (GP) tests for H I, He I, and He II, the proximity ef fect, and Ly alpha emission at high redshifts. It is shown that the io nization state of the diffuse IGM, as required by the GP tests, can be explained without violating any other astrophysical or cosmological c onstraint on the model. We also investigate the implications of recent ly observed resonant neutral helium lines at z similar or equal to 2 o n the radiatively decaying neutrino scenario; this observation rules o ut almost all of the parameter space for neutrino masses greater than or similar to 50 eV.