We report the discovery of deuterium absorption in the very metal-poor ([Fe/H] =−2.88) damped Lyman-α system at z abs = 3.06726 toward the QSO SDSS J1358+6522. On the basis of 13 resolved D i absorption lines and the damping wings of the H i Lyman α transition, we have obtained a new, precise measure of the primordial abundance of deuterium. Furthermore, to bolster the present statistics of precision D/H measures, we have reanalyzed all of the known deuterium absorption-line systems that satisfy a set of strict criteria. We have adopted a blind analysis strategy (to remove human bias), and developed a software package that is specifically designed for precision D/H abundance measurements. For this reanalyzed sample of systems, we obtain a weighted mean of (D / H) p = (2.53 ± 0.04) × 10 −5 , corresponding to a universal baryon density 100 Ω b,0 h 2 = 2.202 ± 0.046 for the standard model of big bang nucleosynthesis (BBN). By combining our measure of (D / H) p with observations of the cosmic microwave background (CMB), we derive the effective number of light fermion species, N eff = 3.28 ± 0.28. We therefore rule out the existence of an additional (sterile) neutrino (i.e. N eff = 4.046) at 99.3 per cent confidence (2.7σ), provided that the values of N eff and of the baryonto-photon ratio (η 10 ) did not change between BBN and recombination. We also place a strong bound on the neutrino degeneracy parameter, independent of the 4 He primordial mass fraction, Y P : ξ D = +0.05 ± 0.13 based only on the CMB+(D / H) p observations. Combining this value of ξ D with the current best literature measure of Y P , we find a 2σ upper bound on the neutrino degeneracy parameter, |ξ| ≤ +0.062.

We report the discovery and analysis of the most metal-poor damped Lyα (DLA) system currently known, which also displays the Lyman series absorption lines of neutral deuterium. cross-section may shed further light on this discrepancy.

We report evidence for a bimodality in damped Ly systems ( DLAs). Using [C ii] 158 m cooling rates, ' c , we find a distribution with peaks at ' c ¼ 10 À27:4 and 10 À26.6 ergs s À1 H À1 separated by a trough at ' crit c % 10 À27:0 ergs s À1 H À1 . We divide the sample into ''low cool'' DLAs with ' c ' crit c and ''high cool'' DLAs with ' c > ' crit c and find the K-S probabilities that velocity width, metallicity, dust-to-gas ratio, and Si ii equivalent width in the two subsamples are drawn from the same parent population are small. These quantities are significantly larger in the high cool population, while the H i column densities are indistinguishable in the two populations. We find the DLA gas is heated by local radiation fields and background radiation, rather than background radiation alone. The rare appearance of faint, extended objects in the Hubble Ultra Deep Field rules out in situ star formation as the dominant star-formation mode for the high cool population, but is compatible with in situ star formation as the dominant mode for the low cool population. Star formation in the high cool DLAs likely arises in Lyman break galaxies. Using Si ii equivalent width as a mass indicator, we construct bivariate distributions of metallicity, ' c , and areal SFR versus the mass indicators. Tentative evidence is found for correlations and parallel sequences, which suggest similarities with the bimodality found in nearby galaxies. We suggest that the transition-mass model provides a plausible scenario for the bimodality we have found. As a result, the bimodality in current galaxies may have originated in DLAs.