The Deuterium Abundance toward Q1937−1009

We present a new measurement of the deuterium-to-hydrogen ratio (D/H) in the Lyman limit absorption system at $z = 3.572$ towards Q1937--1009. We use an improved method to measure D/H in QSO absorption systems, which includes extra parameters to treat the continuum uncertainties, a variety of new absorption models which allow for undetected velocity structure, and the improved measurement of the total hydrogen column density by Burles & Tytler (1997a). We find that all models, including contamination, give an upper limit D/H $< 3.9 \times 10^{-5}$ (95 % confidence). Both this and previous analyses find contamination to be unlikely in this absorption system, A $\chi^2$ analysis in models without contamination gives D/H $= 3.3 \pm 0.3 \times 10^{-5}$ (67% confidence), which is higher but consistent with the earlier results of Tytler et al. (1996), and a second measurement of D/H towards Q1009+2956 (Burles & Tytler 1997). With calculations of standard big bang nucleosynthesis (SBBN) and the assumption that this measurement of D/H is representative of the primordial value, we find a high baryon-to-photon ratio, $\eta = 5.3 \pm 0.4 \times 10^{-10}$. This is consistent with primordial abundance determinations of $^4$He in H II regions (Izotov et al. 1997) and $^7$Li in the atmospheres of warm metal-poor population II stars (Bonifacio & Molaro 1997). We find a high value for the present-day baryon density, $\Omega_b h^2 = 0.0193 \pm 0.0014$, which is consistent with other inventories of baryonic matter, from low to high redshift: clusters of galaxies, the Lyman alpha forest & the Cosmic Microwave Background.Comment: 49 pages, 19 figures, to appear in the Astrophysical Journa