1998ApJ...507..732B

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^+0.06_–0.08. 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^+0.030_–0.035 or D/H = 3.39±0.25x10^–5. Predictions from standard big bang nucleosynthesis give the cosmological baryon-to-photon ratio, η = 5.1±0.3x10^–10, and the baryon density in units of the critical density, Ω_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.