Comparison of stars and decaying neutrinos as additional sources of Intergalactic UV background

Abstract

A numerical calculation of properties of finite absorbers and of intergalactic medium based on photoionization equilibrium is performed to confront alternative UV sources in addition to quasars. It is seen that a spectrum including a large peak around the HI ionization energy due to decaying neutrinos is too soft in the region up to the HeI edge to explain the relatively small observed ratio of neutral He and H densities in Lyman-limit systems if their size is of the kpc order. The recently proposed decrease of the contribution from unstable neutrinos solves this problem but tends to spoil the consistence between IGM and Lyman-$\alpha$ clouds, which requires a large ratio of fluxes for the HI and HeII ionization frequencies, unless there is a very fast decline of quasars above $z = 3$. On the other hand, the addition of stars to quasars may produce a spectrum sufficiently hard between HI and HeI and thereafter soft up to HeII to allow a reasonable agreement of the properties of denser absorbers with those of IGM. This model seems to favour cold dark matter with additional cosmological constant.