The high-redshift deuterium abundance: the z = 3.086 absorption complex towards Q 0420-388

The redshift zabs = 3.08 absorption complex towards the zem = 3.12 quasar 0420 − 388 has high Hi column density components with heavy element abundances ∼ 1/10 solar. A low-redshift component of the complex has Hi column density ∼ 1018 cm−2, and some of the higher order Lyman lines are clean enough to permit a measurement of the deuterium column density for this component. However, while the putative deuterium column density is fairly well-determined, the Hi column density is very uncertain so, if the deuterium identification is correct, the D/H ratio for this component could have any value > 2 × 10−5. The D i/O i ratio is much better constrained, and has a value ∼2. If the Oi/Hi ratio is constant throughout the complex, then D/H ∼ 2 × 10−4.