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

Abstract

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 HI-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 <= 1500 A. 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 <~ 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 Lambda-cosmology with Omega_matter=0.3, Omega_Lambda=0.7 describe the flat absorbers evolution for z <~ 1.0 better than models for Omega_matter=1.0.

arXiv comment9 pages, A&A accepted

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