Spectroscopic analysis reveals wide deuterium-to-hydrogen ratio constraints in a metal-poor gas cloud at redshift 0.701, suggesting complex velocity structure or multiple components.
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 column density to model the absorption. The HST spectrum shows an asymmetric Lyman-$α$ () feature which is produced by either at a second velocity, or a high abundance of D. Three models with a single simple H+D component give 8 × 10⁻⁵ < D/H < 57 × 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 × 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 column densities have more than one component.
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Tytler et al. (1998) studied this question.
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