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Issue A&A
Volume 376, Number 1, September II 2001
Page(s) 1 - 9
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20010944



A&A 376, 1-9 (2001)
DOI: 10.1051/0004-6361:20010944

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

S. Bianchi1, S. Cristiani2, 3 and T.-S. Kim1

1  European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
2  Space Telescope European Coordinating Facility, ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
3  Dipartimento di Astronomia dell'Università di Padova, Vicolo dell'Osservatorio, 35122 Padova, Italy

(Received 11 May 2001 / Accepted 27 June 2001 )

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 $\lambda\ge 1500$ Å. We allow for three values of the fraction of ionising photons that can escape the interstellar medium, $f_{\rm esc}=0.05$, 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 $f_{\rm esc}$, 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 $f_{\rm esc}\la 0.1$ provide an ionising flux compatible with local and high-z determination, including those with a pure QSOs background. The observed $z\sim 1$ break in the evolution can be better explained by a dominant contribution from galaxies. We find that models in $\Lambda$-cosmology with $\Omega_{\rm m}=0.3$, $\Omega_\Lambda=0.7$ describe the flat absorbers evolution for $z\la 1.0$ better than models for $\Omega_{\rm m}=1.0$.


Key words: radiative transfer -- diffuse radiation -- intergalactic medium -- cosmology: theory -- quasar: absorption lines -- ultraviolet: galaxies

Offprint request: S. Bianchi, sbianchi@eso.org




© ESO 2001

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