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The primordial deuterium abundance at z abs = 2.504 from a high signal-to-noise spectrum of Q1009+2956

The spectrum of the z(em) = 2.63 quasar Q1009+2956 has been observed extensively on the Keck telescope. The Lyman limit absorption system z(abs) = 2.504 was previously used to measure D/H by Burles & Tytler using a spectrum with signal to noise (S/N) approximately 60 per pixel in the continuum near Ly alpha at z(abs) = 2.504. The larger data set now available combines to form an exceptionally high S/N spectrum, around 147 per pixel. Several heavy element absorption lines are detected in this Lyman limit system, providing strong constraints on the kinematic structure. We explore a suite of absorption system models and find that the deuterium feature is likely to be contaminated by weak interloping Ly alpha absorption from a low column density HI cloud, reducing the expected D/H precision. We find D/H = 2.48(-0.35)(+0.41) x 10(-5) for this system. Combining this new measurement with others from the literature and applying the method of Least Trimmed Squares to a statistical sample of 15 D/H measurements results in a 'reliable' sample of 13 values. This sample yields a primordial deuterium abundance of (D/H)(p) = (2.545 +/- 0.025) x 10(-5). The corresponding mean baryonic density of the Universe is Omega(b)h(2) = 0.02174 +/- 0.00025. The quasar absorption data are of the same precision as, and marginally inconsistent with, the 2015 CMB Planck (TT+lowP+lensing) measurement, Omega(b)h(2) = 0.02226 +/- 0.00023. Further quasar and more precise nuclear data are required to establish whether this is a random fluctuation.