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, obtained with a long series of exposures with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we deduce a value D/H = (1.65 ± 0.35) × 10-5 (1 σ error) for the abundance of deuterium in the zabs = 2.0762 damped Lyα system (DLA) along this sight line. The velocity structure of this absorber is very simple, and its neutral hydrogen column density N(H I) is accurately known; the error in D/H is mostly due to the limited signal-to-noise ratio of the spectrum. Since this is also one of the most metal-poor DLAs, with metal abundances ~1/200 of solar, the correction due to the astration of D is expected to be insignificant, and the value we deduce should be essentially the primordial abundance of deuterium. When all (six) available measurements of D/H in high-redshift QSO absorbers are considered, we find that the three DLAs, N(H I) is measured most reliably, 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) × 10-5, yields a baryon density ΩBh2 = 0.025 ± 0.001, which is within ~1 σ of the value deduced from the analysis of the cosmic microwave background angular power spectrum, and is still consistent with the present-day D/H and models of Galactic chemical evolution. Future observations of D I absorption in other DLAs are needed to establish whether our finding reflects a real advantage of DLAs over other classes of QSO absorbers for the measurement of D or if it is just a statistical fluctuation.