The Deuterium Abundance toward QSO 1009+2956

Abstract

We present a measurement of the deuterium to hydrogen ratio (D/H) in a metal-poor absorption system at redshift z = 2.504 toward the QSO 1009+2956. We apply the new method of Burles & Tytler 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 +0.06 . This measurement agrees with the low measurement by Burles & Tytler toward Q1937-1009, and the combined value gives the best determination of primordial D/H, log (D/H) p =-4.47 −0.035 +0.030 or D/H = 3.39 ± 0.25 × 10 -5 . Predictions from standard big bang nucleosynthesis give the cosmological baryon-to-photon ratio, η = 5.1 ± 0.3 × 10 -10 , and the baryon density in units of the critical density, Ω b h 2 = 0.019 ± 0.001, where H 0 = 100 h km s -1 Mpc -1 . The measured value of (D/H) p implies that the primordial abundances of both 4 He and 7 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.