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We have modelled the evolution of the number of Ly_alpha absorbers withredshift, resulting from the evolution of the ionising background and theHubble expansion. The contribution of quasars (QSOs) and galaxies to theHI-ionising UV background has been estimated. The QSOs emissivity is derivedfrom recent fits of their luminosity function. The galaxy emissivity iscomputed using a stellar population synthesis model, with a star-formationhistory scaled on observations of faint galaxies at lambda <= 1500 A. We allowfor three values of the fraction of ionising photons that can escape theinterstellar medium, f_esc=0.05, 0.1 and 0.4. The Intergalactic Medium ismodelled as made of purely-absorbing clouds with the distribution in redshiftand column density obtained from QSOs absorption lines. For the adopted valuesof f_esc, the contribution of galaxies to the ionising UV background iscomparable or greater than that of QSOs. Accounting for the contribution ofclouds to the UV emission, all models with f_esc <~ 0.1 provide an ionisingflux compatible with local and high-z determination, including those with apure QSOs background. The observed z ~ 1 break in the evolution can be betterexplained by a dominant contribution from galaxies. We find that models inLambda-cosmology with Omega_matter=0.3, Omega_Lambda=0.7 describe the flatabsorbers evolution for z <~ 1.0 better than models for Omega_matter=1.0.Comment: 9 pages, A&A accepte

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