The contribution of galaxies to the UV ionising background and the evolution of the Lyman forest

We have modelled the evolution of the number of Lyα absorbers with redshift, resulting from the evolution of the ionising background and the Hubble expansion. The contribution of quasars (QSOs) and galaxies to the H i-ionising UV background has been estimated. The QSOs emissivity is derived from recent fits of their luminosity function. The galaxy emissivity is computed using a stellar population synthesis model, with a starformation history scaled on observations of faint galaxies at λ ≥ 1500Å. We allow for three values of the fraction of ionising photons that can escape the interstellar medium, fesc = 0.05, 0.1 and 0.4. The Intergalactic Medium is modelled as made of purely-absorbing clouds with the distribution in redshift and column density obtained from QSOs absorption lines. For the adopted values of fesc, the contribution of galaxies to the ionising UV background is comparable or greater than that of QSOs. Accounting for the contribution of clouds to the UV emission, all models with fesc <∼ 0.1 provide an ionising flux compatible with local and high-z determination, including those with a pure QSOs background. The observed z ∼ 1 break in the evolution can be better explained by a dominant contribution from galaxies. We find that models in Λ-cosmology with Ωm=0.3, ΩΛ=0.7 describe the flat absorbers evolution for z <∼ 1.0 better than models for Ωm=1.0.