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

The redshift z(abs) = 3.08 absorption complex towards the z(em) = 3.12 quasar 0420 - 388 has high H I column density components with heavy element abundances similar to 1/10 solar. A low-redshift component of the complex has H I column density similar to 10(18) 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 H I column density is very uncertain so, if the deuterium identification is correct, the D/H ratio for this component could have any value > 2 x 10(-5). The D I/O I ratio is much better constrained, and has a value similar to 2. If the O I/H I ratio is constant throughout the complex, then D/H similar to 2 x 10(-4).