Clues to the nature of high-redshift O vi absorption systems from their lack of small-scale structure

Abstract

We present results of the first survey of high-redshift $(\langle z\rangle\sim 2.3)$ $\ion{O}{vi}$ absorption systems along parallel lines of sight toward two lensed QSOs. After a careful and well-defined search, we find ten intervening $\ion{O}{vi}$ systems – identified by the presence of the $\lambda\lambda$1031, 1037 doublet lines, $\ion{H}{i}$, and in most cases $\ion{C}{iv}$, $\ion{Si}{iv}$, and $\ion{C}{iii}$ – and eight candidate systems for which we do not detect $\ion{H}{i}$ nor other metals. We assess the veracity of these systems by applying a classification scheme. Within the errors, all $\ion{O}{vi}$ systems appear at the same redshift and have similar line strengths in front of both QSO images, whereas in most cases $\ion{C}{iv}$ or $\ion{Si}{iv}$ show more differences across the lines of sight, either in radial velocity or line strength. We conclude that (1) the coherence length of $\ion{O}{vi}$ must be much larger than $\approx $1 $h_{70}^{-1}$ kpc, and (2) an important fraction of the $\ion{C}{iv}$ absorbers may not reside in the same volume as $\ion{O}{vi}$. Given the inhomogeneous character of the data – different $S/N$ ratios and degrees of blending – we pay special attention to the observational errors and their impact on the above conclusions. Since Doppler parameters are consistent with photoionization, we propose a model in which $\ion{C}{iv}$ occurs in two different photoionized phases, one large, with characteristic sizes of a few hundred kpc and bearing $\ion{O}{vi}$, and another one a factor of ten smaller and containing $\ion{C}{iii}$. This model is able to explain the various transverse differences observed in column density and kinematics. We apply the model successfully to 2 kinds of absorbers, with low and high metallicity. In the low-metallicity regime, [C/H]~ -2, we find that [C/O] ≈ -0.7 is required to explain the observations, which hints at late ($z\la 6$) rather than early metal enrichment. In the high-metallicity regime, the observed dissociation between $\ion{O}{vi}$ and $\ion{C}{iv}$ gas might be produced by galactic outflows. Altogether, the relative abundances, inhomogeneous $\ion{C}{iv}$ and featureless $\ion{O}{vi}$ are consistent with gas that has been processed recently before the absorption occurred (thus close to star-forming regions). Finally, we discuss briefly three associated systems $(z_{\rm abs} \sim z_{\rm em})$ that also show $\ion{O}{vi}$.