2003A&A...397..851L

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 (1996ApJ...461...20H) 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) cannot reproduce the observations, indicating that the stellar contribution to the ionizing background is negligible at z∼3. For the first time a very low carbon abundance of [C/H]=-2.93±0.13 and the abundance ratio [Si/C]=0.35±0.15 are directly measured in the Lyman limit system with N(HI)=3.2x10¹⁷cm^–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≲25M_☉.