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 A are now in agreement with this theory. A new constraint on the metagalactic flux F of Lyman continuum photons at zero redshift, resulting from Hα observations of the intergalactic H I cloud 1225 + 01, leads to a new constraint on the energy E_γ of the decay photons, namely, E_γ = 13.7 ± 0.1 eV, and on the mass m_v of the decaying neutrinos, namely, m_v = 27.4 ± 0.2 eV. We still find Ω～ 1 for the cosmological density parameter. If Ω = 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 Lyα 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.
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