The Deuterium-to-Hydrogen Ratio in a Low-Metallicity Cloud Falling onto the Milky Way

Using Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope observations of the QSO PG 1259+593, we detect D I Lyman series absorption in high-velocity cloud Complex C, a low-metallicity gas cloud falling onto the Milky Way. This is the first detection of atomic deuterium in the local universe in a location other than the nearby regions of the Galactic disk. We construct a velocity model for the sight line based on the numerous O I absorption lines detected in the ultraviolet spectra. We identify eight absorption-line components, two of which are associated with the high-velocity gas in Complex C at ≈-128 and ≈-112 km s-1. A new Westerbork Synthesis Radio Telescope (WSRT) interferometer map of the H I 21 cm emission toward PG 1259+593 indicates that the sight line passes through a compact concentration of neutral gas in Complex C. We use the WSRT data together with single-dish data from the Effelsberg 100 m radio telescope to estimate the H I column density of the high-velocity gas and to constrain the velocity extents of the H I Lyman series absorption components observed by FUSE. We find N(H I) = (9.0 ± 1.0) × 1019 cm-2, N(D I) = (2.0 ± 0.6) × 1015 cm-2, and N(O I) = (7.2 ± 2.1) × 1015 cm-2 for the Complex C gas (68% confidence intervals). The corresponding light-element abundance ratios are D/H = (2.2 ± 0.7) × 10-5, O/H = (8.0 ± 2.5) × 10-5, and D/O = 0.28 ± 0.12. The metallicity of Complex C gas toward PG 1259+593 is approximately 1/6 solar, as inferred from the oxygen abundance [O/H] = -0.79 ±. While we cannot rule out a value of D/H similar to that found for the local ISM (i.e., D/H ∼ 1.5 × 10-5), we can confidently exclude values as low as those determined recently for extended sight lines in the Galactic disk (D/H < 1 × 10-5). Combined with the sub-solar metallicity estimate and the low nitrogen abundance, this conclusion lends support to the hypothesis that Complex C is located outside the Milky Way, rather than inside in material recirculated between the Galactic disk and halo. The value of D/H for Complex C is consistent with the primordial abundance of deuterium inferred from recent Wilkinson Microwave Anisotropy Probe observations of the cosmic microwave background and simple chemical evolution models that predict the amount of deuterium astration as a function of metallicity.