The Neutral Hydrogen Column Density towards Q1937--1009 from the Unabsorbed Intrinsic Continuum in the Lyman-α\alpha Forest

The absorption system at $z=3.572$ towards Q1937--1009 provides the best extragalactic measurement of the atomic deuterium to hydrogen ratio, D/H. We have obtained a new low-resolution, high signal-to-noise ratio (SNR) Keck spectrum to re-measure the total neutral hydrogen column density N(H I) using the amount of Lyman continuum absorption. We develop a new method to directly determine the intrinsic unabsorbed quasar continuum from low-resolution spectra of the \Lya forest for the first time. We use three types of spectra to measure N(H I): (1) A wide slit spectrum for flux calibration, (2) a high-resolution spectrum to determine the unabsorbed continuum between \Lya forest lines, and (3) a high SNR spectrum to measure the residual flux below the Lyman limit. We measure Log [N(H I)] = 17.86 $\pm$ 0.02 \cm2, which is reliable because N(H I) is fully specified by the data. This result is consistent with the N(H I) measured by Tytler, Fan & Burles (1996) from the Lyman series absorption lines, but not with absorption models proposed by Wampler (1996) nor estimates of the total N(H I) by Songaila et al. (1997), both of which suggested lower N(H I) and higher D/H. The Wampler models predict abundant flux below the Lyman limit which is absent from both our old and new spectra, taken with different instruments. The new Keck data is consistent with the data presented by Songaila et al. (1997). The results differ due to the different methods of analysis, and our measurement of the QSO continuum does not agree with the continuum models assumed by Songaila et al. (1997).Comment: 15 pages, 6 figures, To appear in the Astronomical Journal in October 199

Low-Energy Photodisintegration of the Deuteron and Big-Bang Nucleosynthesis

The photon analyzing power for the photodisintegration of the deuteron was measured for seven gamma-ray energies between 2.39 and 4.05 MeV using the linearly polarized gamma-ray beam of the High-Intensity Gamma-ray Source at the Duke Free-Electron Laser Laboratory. The data provide a stringent test of theoretical calculations for the inverse reaction, the neutron-proton radiative capture reaction at energies important for Big-Bang Nucleosynthesis. Our data are in excellent agreement with potential model and effective field theory calculations. Therefore, the uncertainty in the baryon density obtained from Big-Bang Nucleosynthesis can be reduced at least by 20%.Comment: 5 pages, 5 figure