A new measurement of the primordial abundance of deuterium: Toward convergence with the baryon density from the cosmic microwave background?

From the analysis of the near-UV spectrum of the quasar (QSO)2206-199, obta ined with a long series of exposures with the Space Telescope Imaging Spect rograph on the Hubble Space Telescope, we deduce a value D/H = (1.65 +/- 0. 35) x 10(-5) (1 sigma error) for the abundance of deuterium in the z(abs) = 2.0762 damped Ly alpha system (DLA) along this sight line. The velocity st ructure of this absorber is very simple, and its neutral hydrogen column de nsity N(H I) is accurately known; the error in D/H is mostly due to the lim ited signal-to-noise ratio of the spectrum. Since this is also one of the m ost metal-poor DLAs, with metal abundances similar to1/200 of solar, the co rrection due to the astration of D is expected to be insignificant, and the value we deduce should be essentially the primordial abundance of deuteriu m. When all (six) available measurements of D/H in high-redshift QSO absorb ers are considered, we find that the three DLAs, N(H I) is measured most re liably, give consistently lower values than the three Lyman limit systems. We point out that the weighted mean of the DLA measurements, D/H = (2.2 +/- 0.2) x 10(-5), yields a baryon density Omega (B)h(2) = 0.025 +/- 0.001, wh ich is within similar to1 sigma of the value deduced from the analysis of t he cosmic microwave background angular power spectrum, and is still consist ent with the present-day D/H and models of Galactic chemical evolution. Fut ure observations of D I absorption in other DLAs are needed to establish wh ether our finding reflects a real advantage of DLAs over other classes of Q SO absorbers for the measurement of D or if it is just a statistical fluctu ation.