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D. Kirkman (UCSD and Lucent Technologies, Bell Laboratories), D. Tytler (UC San Diego), S. Burles (UCSD and University of Chicago), D. Lubin, J. Omeara (UC San Diego)
We present high signal-to-noise ratio data on an extremely simple zabs ~3 partial Lyman limit absorption system. This absorber has only a single major velocity component, and unsaturated high order H I Lyman series lines. Hence, the parameters which describe the Hydrogen absorption are very well determined. We find excess absorption on the blue wing of the H I Ly\alpha line, near to the expected position of deuterium absorption. The excess absorption is slight, allowing us to set a firm upper limit to the amount of deuterium absorption in this absorber, D/H < 6 \times 10-5. However, the expected deuterium line is at the wrong wavelength to explain all of the excess absorption on the blue wing of the H I Ly\alpha line, and hence there must be contamination by additional H I interloper absorption. Because the parameters of the H I interloper and the D line will be somewhat degenerate, we can not place a lower limit on D/H in this absorption system. We show that this system probably has a very low metallicity, [C/H] < -2, so that our upper limit of D/H < 6 \times 10-5 should be representative of primordial D/H. All four of the known high redshift absorption line systems simple enough to provide useful limits on D are consistent with the low value of D/H = 3.4 ±0.5 \times 10-5 measured towards QSO 1009+2956 and QSO 1937-1009 by Burles and Tytler (1998a, 1998b). Conversely, three of the four D/H limits are inconsistent with high D/H value proposed by Webb et al. (1997a, 1997b) towards QSO 1718+4807.
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