The Deuterium Abundance at z=0.701 towards QSO 1718+4807

TL;DR

This study constrains the deuterium to hydrogen ratio at redshift 0.701 using new spectral data and improved modeling, finding a range of possible D/H values and emphasizing the need for complex models to accurately determine primordial deuterium abundance.

Contribution

It provides updated D/H constraints at z=0.701 using enhanced spectral analysis and modeling, highlighting the importance of velocity components in absorption systems.

Findings

D/H range is 8e-5 to 57e-5 with simple models

Two-component models suggest D/H < 50e-5

Complex velocity structures are necessary for accurate D/H estimates

Abstract

We present constraints on the deuterium to hydrogen ratio (D/H) in the metal-poor gas cloud at redshift $z=0.701$ towards QSO 1718+4807. We use new Keck spectra in addition to Hubble Space Telescope (HST) and International Ultraviolet Explorer (IUE) spectra. We use an improved redshift and a lower \HI column density to model the absorption. The HST spectrum shows an asymmetric Lyman-$\alpha$ (\lya) feature which is produced by either \HI at a second velocity, or a high abundance of D. Three models with a single simple H+D component give $8 \times 10^{-5} < D/H < 57 \times 10^{-5}$ (95%), a much larger range than reported by Webb et al (1997a,b). A more sophisticated velocity distribution, or a second component is necessary for lower D/H. With two components, which could be a part of one absorbing structure, or separate clouds in a galaxy halo, we find $D/H < 50 \times 10^{-5}$. We do not know if this second component is present, but it is reasonable because 40 -- 100% of absorption systems with similar redshifts and \HI column densities have more than one component.