A New Measurement of the Primordial Abundance of Deuterium: Toward Convergence with the Baryon Density from the Cosmic Microwave Background?

Abstract

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 z abs = 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 Ω B h 2 = 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.