We report on detailed Monte Carlo inversion analysis of the Lyman limit system observed at z _ { abs } = 2.917 in the VLT/UVES spectrum of the quasar HE 0940–1050 .
Metal absorption lines of carbon and silicon in three ionization stages and numerous atomic hydrogen lines have been analyzed simultaneously .
It is found that in order to match the observations , the shape of the ultraviolet background ionizing spectrum of Haardt & Madau ( 1996 ) should be modified : a spectrum with a higher intensity of the emission feature at 3 Ryd is required .
It is also found that synthetic galactic spectra ( or different mixtures of them with power law spectra ) can not reproduce the observations , indicating that the stellar contribution to the ionizing background is negligible at z \sim 3 .
For the first time a very low carbon abundance of [ C/H ] = -2.93 \pm 0.13 and the abundance ratio [ Si/C ] = 0.35 \pm 0.15 are directly measured in the Lyman limit system with N ( H i ) = 3.2 \times 10 ^ { 17 } cm ^ { -2 } .
If the absorber at z _ { abs } = 2.917 provides an example of a pristine gas enriched by the nucleosynthetic products of early generations of stars , then the measured value of [ Si/C ] seems to indicate that the initial mass functions for these stellar populations are constrained to intermediate masses , M _ { up } \raise 1.29 pt \hbox { $ < $ \kern - 7.5 pt \raise - 4.73 pt \hbox { $ \sim$ } } 25 M _ % { \odot } .