Extremely metal-poor Lyman limit system at z = 2.917 toward the quasar HE 0940-1050

Abstract

We report on detailed Monte Carlo inversion analysis of the Lyman limit system observed at z = 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) 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(H I) = 3.2 10^{17} cm^{-2}. If the absorber at z = 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_solar.

arXiv comment7 pages, 4 figures, A&A in press

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• The most metal-poor damped Ly alpha system at z<3: constraints on early nucleosynthesis
• Spectral energy distribution of the metagalactic ionizing radiation field from QSO absorption spectra
• He II absorption and the sawtooth spectrum of the cosmic far-UV background
• Possible Population III Remnants at Redshift 3.5
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• Predominantly Low Metallicities Measured in a Stratified Sample of Lyman Limit Systems at z=3.7
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• Metal-rich absorbers at high redshifts: abundance patterns
• Quasar spectral energy distribution in EUV restored from associated absorbers: indications to the HeII opacity of the quasar accretion disk wind