Precision Measures of the Primordial Abundance of Deuterium

Abstract

We report the discovery of deuterium absorption in the very metal-poor ([Fe/H] = -2.88) damped Lyα system at z _abs = 3.06726 toward the QSO SDSS J1358+6522. On the basis of 13 resolved D I absorption lines and the damping wings of the H I Lyα transition, we have obtained a new, precise measure of the primordial abundance of deuterium. Furthermore, to bolster the present statistics of precision D/H measures, we have reanalyzed all of the known deuterium absorption-line systems that satisfy a set of strict criteria. We have adopted a blind analysis strategy (to remove human bias) and developed a software package that is specifically designed for precision D/H abundance measurements. For this reanalyzed sample of systems, we obtain a weighted mean of (D/H)_p = (2.53 ± 0.04) × 10^-5, corresponding to a universal baryon density 100 Ω_{b, 0} h ² = 2.202 ± 0.046 for the standard model of big bang nucleosynthesis (BBN). By combining our measure of (D/H)_p with observations of the cosmic microwave background (CMB), we derive the effective number of light fermion species, N _eff = 3.28 ± 0.28. We therefore rule out the existence of an additional (sterile) neutrino (i.e., N _eff = 4.046) at 99.3% confidence (2.7σ), provided that the values of N _eff and of the baryon-to-photon ratio (η₁₀) did not change between BBN and recombination. We also place a strong bound on the neutrino degeneracy parameter, independent of the ⁴He primordial mass fraction, Y _P: ξ_D = +0.05 ± 0.13 based only on the CMB+(D/H)_p observations. Combining this value of ξ_D with the current best literature measure of Y _P, we find a 2σ upper bound on the neutrino degeneracy parameter, |ξ| <= +0.062.

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (VLT program IDs: 68.B-0115(A), 70.A-0425(C), 078.A-0185(A), 085.A-0109(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. Keck telescope time was partially granted by NOAO, through the Telescope System Instrumentation Program (TSIP). TSIP is funded by NSF.