The Deuterium Abundance toward 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 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 H1. 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. 06-₀. 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^-5. Predictions from standard big bang nucleosynthesis (SBBN) give the cosmological baryon to photon ratio, = 5. 1 0. 3 10^-10, and the baryon density in units of the critical density, b h² = 0. 019 0. 001, where H₀ = 100 h Mpc^-1. The measured value of (D/H) ₚ implies that the primordial abundances of both ⁴He and ⁷Li 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.