The Primordial Deuterium Abundance of the Most Metal-poor Damped Lyman-α System

We report the discovery and analysis of the most metal-poor damped Lyα (DLA) system currently known, which also displays the Lyman series absorption lines of neutral deuterium. The average [O/H] abundance of this system is [O/H] = -2.804 ± 0.015, which includes an absorption component with [O/H] = -3.07 ± 0.03. Despite the unfortunate blending of many weak D I absorption lines, we report a precise measurement of the deuterium abundance of this system. Using the six highest-quality and self-consistently analyzed measures of D/H in DLAs, we report tentative evidence for a subtle decrease of D/H with increasing metallicity. This trend must be confirmed with future high-precision D/H measurements spanning a range of metallicity. A weighted mean of these six independent measures provides our best estimate of the primordial abundance of deuterium, 10⁵ (D/H)_P = 2.547 ± 0.033 ({{log}}₁₀ {{{(D/H)}}}_{{P}}=-4.5940+/- 0.0056). We perform a series of detailed Monte Carlo calculations of Big Bang nucleosynthesis (BBN) that incorporate the latest determinations of several key nuclear reaction cross-sections, and propagate their associated uncertainty. Combining our measurement of (D/H)_P with these BBN calculations yields an estimate of the cosmic baryon density, 100 Ω_B,0 h ²(BBN) = 2.156 ± 0.020, if we adopt the most recent theoretical determination of the d{(p,γ )}³{He} reaction rate. This measure of Ω_B,0 h ² differs by ∼2.3σ from the Standard Model value estimated from the Planck observations of the cosmic microwave background. Using instead a d{(p,γ )}³{He} reaction rate that is based on the best available experimental cross-section data, we estimate 100 Ω_B,0 h ²(BBN) = 2.260 ± 0.034, which is in somewhat better agreement with the Planck value. Forthcoming measurements of the crucial d{(p,γ )}³{He} cross-section may shed further light on this discrepancy.

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (VLT program ID: 093.A-0016(A)), and at the W.M. Keck Observatory which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.