The Deuterium Abundance toward QSO 1009+2956

We present a measurement of the deuterium to hydrogen ratio (D/H) in ametal-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 inhigh resolution \Lya forest spectra, and include a constraint on the neutralhydrogen column density determined from the Lyman continuum optical depth inlow resolution spectra. We introduce six separate models to measure D/H and toassess the systematic dependence on the assumed underlying parameters. We findthat the deuterium absorption feature contains a small amount of contaminationfrom unrelated \ion{H}{1}. Including the effects of the contamination, wecalculate 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 lowmeasurement by Burles & Tytler (1997) towards Q1937--1009, and the combinedvalue gives the best determination of primordial D/H, log (D/H)$_p = -4.47^{+0.030}_{-0.035}$ or D/H $= 3.39 \pm 0.25 \times 10^{-5}$. Predictions fromstandard big bang nucleosynthesis (SBBN) give the cosmological baryon to photonratio, $\eta = 5.1 \pm 0.3 \times 10^{-10}$, and the baryon density in units ofthe critical density, $\Omega_b h^2 = 0.019 \pm 0.001$, where $H_0 = 100 h$\kms Mpc$^{-1}$. The measured value of (D/H)$_p$ implies that the primordialabundances of both $^4$He and $^7$Li are high, and consistent with some recentstudies. Our two low measurements of primordial D/H also place strongconstraints on inhomogeneous models of big bang nucleosynthesis.Comment: 47 pages, 24 figures, submitted to the Astrophysical Journa