SIMBAD references

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 16 quasars at z_QSO between 2.14 and 2.87 observed at high signal-to-noise ratio and 6.6 km/s resolution with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph (VLT/UVES), we detect 35 Ovi absorption-line systems (comprising over 100 individual Ovi components) lying within 8000km/s of z_QSO. We present component fits to the Ovi absorption and the accompanying Hi, Civ and Nv. The systems can be categorized into nine 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 quasar-driven outflows or gas close to the central engine of the active galactic nuclei. 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 42km/s. The incidence dN/dz of weak Ovi systems within 2000km/s of the quasar down to a limiting equivalent width of 8 mÅ is 42±12. Between 2000 and 8000km/s, dN/dz falls to 14 ±4, equal to the incidence of intervening Ovi absorbers measured in the same spectra. Whereas the accompanying Hi and Civ column densities are significantly lower (by a mean of ∼1 dex) in the weak Ovi absorbers within 2000 km/s of z_QSO than in those at larger velocities, the Ovi column densities display no dependence on proximity. Furthermore, significant offsets between the Hi and Ovi centroids in ~50 per cent of the weak absorbers imply that (at least in these cases) the Hi 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 multiphase haloes 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 < 14km/s) in over half of the observed Ovi components.