2005A&A...441....9A

A computational procedure is presented to estimate the spectral shape of the ionizing background between 1 and 10Ryd by analyzing optically thin absorption systems in the spectra of high redshift quasars. The procedure is based on the response surface methodology from the theory of experimental design. The shape of the recovered UV background at z∼3 shows a significant intensity decrease between 3 and 4Ryd compared to the metagalactic spectrum of Haardt & Madau (1996ApJ...461...20H). This decrease is interpreted as produced by HeII Gunn-Peterson effect. There are no features indicating a contribution from galaxies to the UV background which is therefore dominated by QSOs at z∼3.