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 Ly? 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, ? = 5.1 ? 0.3 ? 10-10, and the baryon density in units of the critical density, ?bh2 = 0.019 ? 0.001, where H0 = 100h km s-1 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.