The Deuterium Abundance towards QSO 1009 + 2956

We present a measurement of the deuterium to hydrogen ratio (D/H) in a metal-poor absorption system at redshift z = 2.504 towards the QSO 1009+2956. We apply the new method of Burles & Tytler (1997) to robustly determine D/H in high resolution Lyα forest spectra, and include a constraint on the neutral hydrogen column density determined from the Lyman continuum optical depth in low resolution spectra. We introduce six separate models to measure D/H and to assess the systematic dependence on the assumed underlying parameters. We find that the deuterium absorption feature contains a small amount of contamination from unrelated H I. Including the effects of the contamination, we calculate the 67% confidence interval of D/H in this absorption system, log (D/H) = −4.40 −0.08. This measurement agrees with the low measurement by Burles & Tytler (1997) towards Q1937–1009, and the combined value gives the best determination of primordial D/H, log (D/H)p = −4.47 +0.030 −0.035 or D/H = 3.39 ± 0.25 × 10. Predictions from standard big bang nucleosynthesis (SBBN) give the cosmological baryon to photon ratio, η = 5.1 ± 0.3 × 10, and the baryon density in units of the critical density, Ωb h 2 = 0.019 ± 0.001, where H0 = 100 h km s −1 Mpc. The measured value of (D/H)p implies that the primordial abundances of both He and Li are high, and consistent with some recent studies. Our two low measurements of primordial D/H also place strong constraints on inhomogeneous models of big bang nucleosynthesis. Visiting Astronomer, W. M. Keck Telescope, California Association for Research in Astronomy.