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 red shift, resulting from the evolution of the ionising background and the Hubb le expansion. The contribution of quasars (QSOs) and galaxies to the H I-io nising UV background has been estimated. The QSOs emissivity is derived fro m recent fits of their luminosity function. The galaxy emissivity is comput ed using a stellar population synthesis model, with a star-formation histor y 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 pho tons that can escape the interstellar medium, f(esc) = 0.05, 0.1 and 0.4. T he Intergalactic Medium is modelled as made of purely-absorbing clouds with the distribution in redshift and column density obtained from QSOs absorpt ion lines. For the adopted values of f(esc), the contribution of galaxies t o the ionising UV background is comparable or greater than that of QSOs. Ac counting 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 wit h local and high-z determination, including those with a pure QSOs backgrou nd. The observed z similar to 1 break in the evolution can be better explai ned 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 abso rbers evolution for z less than or similar to 1.0 better than models for Om ega (m) = 1.0.