A Study of Quasar Proximity in Ovi absorbers at z=2–3⋆

With the goal of investigating the nature of Ovi absorbers at high redshifts, we study the effects of proximity to the background quasar. In a sample of sixteen quasars at zqso between 2.14 and 2.87 observed at high signal-to-noise and 6.6 km s −1 resolution with VLT/UVES, we detect 35 Ovi absorption-line systems (comprising over 100 individual Ovi components) lying within 8 000 km s of zqso. We present component fits to the Ovi absorption and the accompanying H i, C iv, and Nv. The systems can be categorized into 9 strong and 26 weak Ovi absorbers. The strong (intrinsic) absorbers are defined by the presence of either broad, fully saturated Ovi absorption or partial coverage of the continuum source, and in practice all have logN(Ovi)&15.0; these systems are interpreted as representing either QSO-driven outflows or gas close the central engine of the AGN. The weak (also known as narrow) systems show no partial coverage or saturation, and are characterized by logN(Ovi)<14.5 and a median total velocity width of only 42 kms. The incidence dN/dz of weak Ovi systems within 2 000 km s of the quasar down to a limiting equivalent width of 8 mÅ is 42±12. Between 2 000 and 8 000 km s, dN/dz falls to 14±4, equal to the incidence of intervening Ovi absorbers measured in the same spectra. Whereas the accompanying H i and C iv column densities are significantly lower (by a mean of ∼1 dex) in the weak Ovi absorbers within 2 000 km s of zqso than in those at larger velocities, the Ovi column densities display no dependence on proximity. Furthermore, significant offsets between the H i and Ovi centroids in ≈50 per cent of the weak absorbers imply that (at least in these cases) the H i and Ovi lines are not formed in the same phase of gas, preventing us from making reliable metallicity determinations, and ruling out single-phase photoionization-model solutions. In summary, we find no firm evidence that quasar radiation influences the weak Ovi absorbers, suggesting they are collisionally ionized rather than photoionized, possibly in the multi-phase halos of foreground galaxies. Non-equilibrium collisional ionization models are needed to explain the low temperatures in the absorbing gas, which are implied by narrow line widths (b<14 km s) in over half of the observed Ovi components.