Highly ionised absorbers at high redshift

We build a sample of O VI absorption systems in the redshift range 2.0 ≲ z ≲ 2.6 using high spectral resolution data of ten quasars from the VLT-UVES large programme. We investigate the existence of a metal-rich O VI population and define observational criteria for this class of absorbers under the assumption of photoionisation. The low temperatures of nearly half of all O VI absorbers, implied by their line widths, are too low for collisional ionisation to be a dominant process. We estimate the oxygen abundance under the assumption of photoionisation; a striking result is the bimodal distribution of [o/h] with median values close to 0.01 and 0.5 solar for the metal-poor and metal-rich populations, respectively. Using the line widths to fix the temperature or assuming a constant, low gas density does not drastically change the metallicities of the metal-rich population. We present the first estimate of the O VI column density distribution. Assuming a single power-law distribution, f(n) ∝ n^-α, yields α ∼ 1.7 and a normalisation of f(n) =2.3× 10^-13 at log n(O VI) ∼ 13.5, both with a ∼30% uncertainty. The value of α is similar to that found for C IV surveys, whereas the normalisation factor is about ten times higher. We use f(n) to derive the number density per unit z and cosmic density ω_b(O VI), selecting a limited column density range not strongly affected by incompleteness or sample variance. Comparing our results with those obtained at z∼0.1 for a similar range of column densities implies some decline of dn/dz with z. The cosmic O VI density derived from f(n), ω_b(O VI)≈ (3.5± ^3.2_0.9) × 10^-7, is 2.3 times higher than the value estimated using the observed O VI sample (of which the metal-rich population contributes ∼35%), easing the problem of missing metals at high z (∼ 1/4 of the produced metals) but not solving it. We find that the majori ty of the metal-rich absorbers are located within ∼ 450 km s^-1 of strong Ly-α lines and show that, contrary to the metal-poor absorbers, this population cannot be in hydrostatic equilibrium. All of the O VI absorber properties imply that there are two distinct populations: metal-poor absorbers tracing the intergalactic medium and metal-rich absorbers associated with active sites of star formation and most probably linked to galactic winds.