High-resolution OVI absorption line observations at 1.2 <= z <= 1.7 in thebright QSO HE 0515-4414

STIS Echelle observations at a resolution of 10 km s(-1) and UVES/VLT spect roscopy at a resolution of 7 km s(-1) of the luminous QSO HE 0515-4414 (z(e m) = 1.73, B = 15.0) reveal four intervening O VI absorption systems in the redshift range 1.21 less than or equal to z(abs) less than or equal to 1.6 7 (1.38503, 1.41601, 1.60175, 1.67359). In addition, two associated systems at z = 1.69707 and z = 1.73585 are present. Noteworthy is an absorber at z = 1.385 with log N-HI = 13.9 and strong O VI (N(O VI)/N(H I) approximate t o 1) and C IV doublets, while a nearby much stronger Ly alpha absorber (log N-HI = 14.8, Deltav = 123 km s(-1)) does not reveal any heavy element abso rption. For the first time, high resolution observations allow one to measu re radial velocities of H I, C IV and O VI simultaneously in several absorp tion systems (1.385, 1.674, 1.697) with the result that significant velocit y differences (up to 18 km s(-1), are observed between H I and O VI, while smaller differences (up to 5 km s(-1)) are seen between C IV and O VI. We t entatively conclude that H I, O VI, and C IV are not formed in the same vol umes and that therefore conclusions on ionization mechanisms are not possib le from the observed column density ratios O VI/H I or O VI/C IV. The numbe r density of O VI absorbers with W-rest greater than or equal to 25 m Angst rom is dN/dz less than or equal to 10, roughly a factor of 5 less than that found by Tripp et al. (2000) at low redshift. However, this number is unce rtain and further lines of sight will be probed in the next HST cycle. An e stimate of the cosmological mass-density of the O VI-phase yields Omega (b) (O VI) approximate to 0.0003 h(75)(-1) for [O/H] = -1 and an assumed ioniza tion fraction O VI/O = 0.2. It should be noted that this result is subject to large systematic errors. This corresponds to an increase by roughly a fa ctor of 15 between (z) over bar = 1.5 (this work) and the value found by Tr ipp et al. (2000) at (z) over bar = 0.21, if the same oxygen abundance [O/H ] = -1 is assumed. Agreement with the simulations by Dave et al. (2001) can be obtained, if the oxygen abundance increases by a factor of similar to 3 over the same redshift interval.