COMPARISON OF STARS AND DECAYING NEUTRINOS AS ADDITIONAL SOURCES OF THE INTERGALACTIC UV BACKGROUND

A numerical calculation of properties of finite absorbers and of inter galactic medium based on photoionization equilibrium is performed to c onfront 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 densi ties in Lyman-limit systems if their size is of the kpc order. The rec ently proposed decrease of the contribution from unstable neutrinos so lves this problem but tends to spoil the consistence between IGM and L yman-alpha clouds, which requires a large ratio of fluxes for the HI a nd HeII ionization frequencies, unless there is a very fast decline of quasars above z = 3. On the other hand, the addition of stars to quas ars may produce a spectrum sufficiently hard between HI and HeI and th ereafter soft up to HeII to allow a reasonable agreement of the proper ties of denser absorbers with those of IGM. This model seems to favour cold dark matter with additional cosmological constant.