This study focuses on some of the most metal-poor damped Lyα (DLA) absorbers known in the spectra of high-redshift QSOs, using new and archival observations obtained with ultravioletsensitive echelle spectrographs on the Keck and VLT telescopes. The weakness and simple velocity structure of the absorption lines in these systems allow us to measure the abundances of several elements, and in particular those of C, N and O, a group that is difficult to study in DLAs of more typical metallicities. We find that when the oxygen abundance is less than ∼1/100 of solar, the C/O ratio in high-redshift DLAs and sub-DLAs matches that of halo stars of similar metallicity and shows higher values than expected from galactic chemical evolution models based on conventional stellar yields. Furthermore, there are indications that at these low metallicities the N/O ratio may also be above simple expectations and may exhibit a minimum value, as proposed by Centurión and her collaborators in 2003. Both results can be interpreted as evidence for enhanced production of C and N by massive stars in the first few episodes of star formation, in our Galaxy and in the distant protogalaxies seen as QSO absorbers. The higher stellar yields implied may have an origin in stellar rotation which promotes mixing in the stars' interiors, as considered in some recent model calculations. We briefly discuss the relevance of these results to current ideas on the origin of metals in the intergalactic medium and the universality of the stellar initial mass function.

The most metal-poor damped Lyα system known to date, at z abs = 2.61843 in the spectrum of the QSO Q0913+072, with an oxygen abundance of only ∼1/250 of the solar value, shows six well-resolved D I Lyman series transitions in high-quality echelle spectra recently obtained with the European Southern Observatory (ESO) VLT. We deduce a value of the deuterium abundance log (D/H) = −4.56 ± 0.04 which is in good agreement with four out of the six most reliable previous determinations of this ratio in QSO absorbers. We find plausible reasons why in the other two cases the 1σ errors may have been underestimated by about a factor of two. The addition of this latest data point does not significantly change the mean value of the primordial abundance of deuterium, suggesting that we are now converging to a reliable measure of this quantity. We conclude that log (D/H) p = −4.55 ± 0.03 and b,0 h 2 (BBN) = 0.0213 ± 0.0010 (68 per cent confidence limits). Including the latter as a prior in the analysis of the Wilkinson Microwave Anisotropy Probe (WMAP) five-year data leads to a revised best-fitting value of the power-law index of primordial fluctuations n s = 0.956 ± 0.013 (1σ ) and n s < 0.990 with 99 per cent confidence. Considering together the constraints provided by WMAP 5, (D/H) p , baryon oscillations in the galaxy distribution, and distances to Type Ia supernovae, we arrive at the current best estimates b,0 h 2 = 0.0224 ± 0.0005 and n s = 0.959 ± 0.013.

We present the results of the largest survey to date for intergalactic metals at redshifts z > 5, using near‐IR spectra of nine quasi‐stellar objects with emission redshifts zem > 5.7. We detect three strong C iv doublets at zabs= 5.7–5.8, two low‐ionization systems at zabs > 5 and numerous Mg ii absorbers at zabs= 2.5–2.8. We find, for the first time, a change in the comoving mass density of C iv ions as we look back to redshifts z > 5. At a mean 〈z〉= 5.76, we deduce ΩC iv= (4.4 ± 2.6) × 10−9 which implies a drop by a factor of ∼3.5 compared to the value at z < 4.7, after accounting for the differing sensitivities of different surveys. The observed number of C iv doublets is also lower by a similar factor, compared to expectations for a non‐evolving column density distribution of absorbers. These results point to a rapid build‐up of intergalactic C iv over a period of only ∼300 Myr; such a build‐up could reflect the accumulation of metals associated with the rising levels of star formation activity from z∼ 9 indicated by galaxy counts, and/or an increasing degree of ionization of the intergalactic medium (IGM), following the overlap of ionization fronts from star‐forming regions. If the value of ΩC iv we derive is typical of the IGM at large, it would imply a metallicity ZIGM≳ 10−4 Z⊙. The early‐type stars responsible for synthesizing these metals would have emitted only about one Lyman continuum photon per baryon prior to z= 5.8; such a background is insufficient to keep the IGM ionized and we speculate on possible factors which could make up the required shortfall.

A correlation between the metallicity, [M/H], and rest-frame Mg II equivalent width, W λ2796 r