The Deuterium-to-Hydrogen Abundance Ratio toward the QSO SDSS J155810.16-003120.0

We present a measurement of the D/H abundance ratio in a metal-poor dampedLyman alpha (DLA) system along the sightline of QSO SDSS1558-0031. The DLAsystem is at redshift z = 2.70262, has a neutral column density oflog(NHI)=20.67+/-0.05 cm^2, and a gas-phase metallicity [O/H]= -1.49 whichindicates that deuterium astration is negligible. Deuterium absorption isobserved in multiple Lyman series with a column density oflog(NDI)=16.19+/-0.04 cm^2, best constrained by the deuterium Lyman-11 line. Wemeasure log(D/H) = -4.48+/-0.06, which when combined with previous measurementsalong QSO sightlines gives a best estimate of log(D/H) = -4.55+/-0.04, wherethe 1-sigma error estimate comes from a jackknife analysis of the weightedmeans. Using the framework of standard big bang nucleosynthesis, this value ofD/H translates into a baryon density of Omega_b h^2 = 0.0213 +/- 0.0013 +/-0.0004 where the error terms represent the 1-sigma errors from D/H and theuncertainties in the nuclear reaction rates respectively. Combining our newmeasurement with previous measurements of D/H, we no longer find compellingevidence for a trend of D/H with NHI.Comment: 13 pages, 3 figures, 1 table. Accepted to the Astrophysical Journal Letter