QSO 0130−4021: A Third QSO Showing a Low Deuterium‐to‐Hydrogen Abundance Ratio

We have discovered a third quasar absorption system which is consistent with a low deuterium-to-hydrogen abundance ratio, D/H = 3.4 × 10 -5 . The z abs ~ 2.8 partial Lyman limit system toward Q0130-4021 provides the strongest evidence to date against large D/H ratios because the H I absorption, which consists of a single high column density component with unsaturated high-order Lyman series lines, is readily modeled—a task which is more complex in other D/H systems. We have obtained 22 hr of spectra from the High-Resolution Echelle Spectrograph on the W. M. Keck Telescope, which allow a detailed description of the hydrogen. We see excess absorption on the blue wing of the H I Lyα line, near the expected position of deuterium. However, we find that deuterium cannot explain all of the excess absorption, and hence there must be contamination by additional absorption, probably H I. This extra H I can account for most or all of the absorption at the D position, and hence D/H = 0 is allowed. We find an upper limit of D/H ≤ 6.7 × 10 -5 in this system, consistent with the value of D/H ≃ 3.4 × 10 -5 deduced toward QSO 1009+2956 and QSO 1937-1009 by Burles and Tytler. This absorption system shows only weak metal-line absorption, and we estimate [Si/H] ≤ -2.6, indicating that the D/H ratio of the system is likely primordial. All four of the known high-redshift absorption-line systems simple enough to provide useful limits on D are consistent with D/H = 3.4 ± 0.25 × 10 -5 . Conversely, this QSO provides the third case which is inconsistent with much larger values.