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

We present results of the first survey of high-redshift (< z > similar to 2.3) O VI absorption systems along parallel lines of sight toward two lensed QSOs. After a careful and well-defined search, we find ten intervening O VI systems - identified by the presence of the lambda lambda 1031, 1037 doublet lines, Hi, and in most cases C IV, Si IV, and C III - and eight candidate systems for which we do not detect H I nor other metals. We assess the veracity of these systems by applying a classification scheme. Within the errors, all O VI systems appear at the same redshift and have similar line strengths in front of both QSO images, whereas in most cases C IV or 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 O VI must be much larger than approximate to 1 h(70)(-1) kpc, and (2) an important fraction of the C IV absorbers may not reside in the same volume as 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 C IV occurs in two different photoionized phases, one large, with characteristic sizes of a few hundred kpc and bearing O VI, and another one a factor of ten smaller and containing 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]similar to -2, we find that [C/O]similar to -0.7 is required to explain the observations, which hints at late (z less than or similar to 6) rather than early metal enrichment. In the high-metallicity regime, the observed dissociation between O VI and C IV gas might be produced by galactic outflows. Altogether, the relative abundances, inhomogeneous C IV and featureless 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(abs) similar to z(em)) that also show O VI.