DECAYING NEUTRINOS, THE METAGALACTIC RADIATION-FIELD, AND THE IONIZATION OF NITROGEN IN THE INTERSTELLAR-MEDIUM

The decaying neutrino theory for the ionization of the interstellar medium is reevaluated in the light of recent observational results. Estimates of the isotropic component of the far-UV background at 1500 Angstrom are now in agreement with this theory. A new constraint on the metagalactic flux F of Lyman continuum photons at zero redshift, resulting from Ha observations of the intergalactic H I cloud 1225+01, leads to a new constraint on the energy E(gamma) of the decay photons, namely, E(gamma) = 13.7 +/- 0.1 eV, and on the mass m(nu) of the decaying neutrinos, namely, m(nu) = 27.4 +/- 0.2 eV. We still find Omega similar to 1 for the cosmologicaI density parameter. If Omega = 1 exactly, then the Hubble constant H-0 = 54 +/- 0.5 km s(-1) Mpc(-1). The new constraint on F is still compatible with the hypothesis that decay photons are mainly responsible for the ionization of hydrogen in Ly alpha clouds and the intergalactic medium at both small and large redshifts. However, decay photons can no longer ionize nitrogen, so the origin of the widespread distribution of N II observed in the Galaxy and in NGC 891 must be reconsidered. It is shown that in the presence of the proposed galactic flux F-g of decay photons, an additional flux of conventional ionizing photons of only 1% of F-g would suffice to account for the observed distribution of N II.