The deuterium abundance in the z = 0.7 absorber towards QSO PG1718+4807

We report on a further analysis of the ratio of deuterium to hydrogen (D/H) using Hubble Space Telescope (HST) spectra of the z = 0.701 Lyman limit system towards the quasi-stellar object (QSO) PG1718+481. Initial analyses of this absorber found it gave a high D/H value, 1.8-3.1 × 10^-4, inconsistent with several higher redshift measurements. It is thus important to critically examine this measurement. By analysing the velocity widths of the D I, H I and metal lines present in this system, Kirkman et al. report that the additional absorption in the blue wing of the Lyα line cannot be D I, with a confidence level of 98 per cent. Here we present a more detailed analysis, taking into account possible wavelength shifts between the three sets of HST spectra used in the analysis. We find that the constraints on this system are not as strong as those claimed by Kirkman et al. The discrepancy between the parameters of the blue wing absorption and the parameters expected for D I is marginally worse than 1 σ. Tytler et al. commented on the first analysis of Webb et al., reporting the presence of a contaminating lower redshift Lyman limit system, with log[Af(H I)] = 16.7 at z = 0.602, which biases the N(H I) estimate for the main system. Here we show that this absorber actually has log[W(H I)] < 14.6 and does not impact on the estimate of Af(H I) in the system of interest at z = 0.701. The purpose of the present paper is to highlight important aspects of the analysis which were not explored in previous studies, and hence to help refine the methods used in future analyses of D/H in quasar spectra.