The Deuterium Abundance Towards Q 1937 – 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. Tytler, Fan & Burles (1996; hereafter TFB) made the first extragalactic detection of deuterium in this absorption system, which remains the best location for a high accuracy measurement of primordial D/H. Their detailed analysis of Keck spectra gave a low value of D/H = 2.3 ± 0.3 ± 0.3 × 10 (1σ statistical & systematic errors). Now we present a new method to measure D/H in QSO absorption systems. We avoid many of the assumptions adopted by TFB; we allow extra parameters to treat the continuum uncertainties, include a variety of new absorption models which allow for undetected velocity structure, and use 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 × 10 (95 % confidence). Both this and previous analyses find contamination to be unlikely in this absorption system, A χ analysis in models without contamination gives D/H = 3.3± 0.3× 10 (67% confidence), which is higher but consistent with the earlier results of TFB, and a second measurement of D/H towards Q1009+2956 (Tytler & Burles 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, η = 5.3 ± 0.4 × 10. This is consistent with primordial abundance determinations of He in H II regions (Izotov et al. 1997) Visiting Astronomer, W. M. Keck Telescope, California Association for Research in Astronomy Present Address: University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 Visiting Associate, European Southern Observatory.