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 witha low deuterium to hydrogen abundance ratio, D/H = 3.4 times 10^-5. The z ~ 2.8partial Lyman limit system towards QSO 0130-4021 provides the strongestevidence to date against large D/H ratios because the H I absorption, whichconsists of a single high column density component with unsaturated high orderLyman series lines, is readily modeled -- a task which is more complex in otherD/H systems. We have obtained twenty-two hours of spectra from the HIRESspectrograph on the W.M. Keck telescope, which allow a detailed description ofthe Hydrogen. We see excess absorption on the blue wing of the H I Lyman alphaline, near the expected position of Deuterium. However, we find that Deuteriumcannot explain all of the excess absorption, and hence there must becontamination by additional absorption, probably H I. This extra H I canaccount for most or all of the absorption at the D position, and hence D/H = 0is allowed. We find an upper limit of D/H < 6.7 times 10^-5 in this system,consistent with the value of D/H ~ 3.4 times 10^-5 deduced towards QSO1009+2956 and QSO 1937-1009 by Burles and Tytler (1998a, 1998b). Thisabsorption 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. Allfour of the known high redshift absorption line systems simple enough toprovide useful limits on D are consistent with D/H = 3.4 +/- 0.25 times 10^-5.Conversely, this QSO provides the third case which is inconsistent with muchlarger values.Comment: 18 pages, 5 figures, submitted to Ap