2003ApJS..149....1K

We report the detection of deuterium absorption at redshift 2.525659 toward Q1243+3047. We describe improved methods to estimate the deuterium to hydrogen abundance ratio (D/H) in absorption systems, including improved modeling of the continuum level, the Lyα forest, and the velocity structure of the absorption. Together with improved relative flux calibration, these methods give D/H=2.42^+0.35_–0.25x10^–5 from our Keck I HIRES spectra of Q1243+3047, where the error is from the uncertainty in the shape of the continuum level and the amount of D absorption in a minor second component. The measured D/H is likely the primordial value because [O/H]=-2.79±0.05. This absorption system has a neutral hydrogen column density logN_HI_=19.73±0.04 cm^–2, shows five D lines, and is mostly ionized. The best estimate of the primordial D/H is 2.78^+0.44_–0.38x10^–5, from the logD/H-values toward five QSOs. The dispersion in the five values is larger than we expect from their individual measurement errors, and we suspect this is because some of these errors were underestimated. We observe a trend in D/H with logN_HI_that we also suspect is spurious. The best value for D/H is 0.6 σ smaller than we quoted in O'Meara et al. from three QSOs, and although we have more values, the error is similar because the dispersion is larger. In standard big bang nucleosynthesis (SBBN), the best D/H corresponds to a baryon-to-photon ratio η=5.9±0.5x10^–10 and gives precise predictions for the primordial abundances of the other light nuclei. We predict more ⁴He than is reported in most measurements, although not more than allowed by some estimates of the systematic errors. We predict a ³He abundance very similar to that reported by Bania et al., and we predict 3-4 times more ⁷Li than is seen in halo stars. It is unclear if those stars could have destroyed this much of their ⁷Li. The η-value from D/H corresponds to a cosmological baryon density Ω_bh²=0.0214±0.0020 (±9.3%), which agrees with the WMAP value of Ω_bh²=0.0224±0.001.