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 Ly alpha 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 star-formation history scaled on observations of faint galaxies at lambda greater than or equal to 1500 Angstrom. We allow for three values of the fraction of ionising photons that can escape the interstellar medium, f(esc) = 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 f(esc), 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 f(esc) less than or similar to 0.1 provide an ionising flux compatible with local and high-z determination, including those with a pure QSOs background. The observed z similar to 1 break in the evolution can be better explained by a dominant contribution from galaxies. We find that models in Lambda -cosmology with Omega (m) = 0.3, Omega (Lambda) = 0.7 describe the at absorbers evolution for z less than or similar to 1.0 better than models for Omega (m) = 1.0.