High-resolution OVI absorption line observations at 1.2 ≤ z ≤ 1.7 in the bright QSO HE 0515-4414

STIS Echelle observations at a resolution of 10 km s(-1) and UVES/VLT spectroscopy at a resolution of 7 km s(-1) of the luminous QSO HE 0515-4414 (z(em) = 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 1.67 (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 to 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 absorption. For the first time, high resolution observations allow one to measure radial velocities of H I, C IV and O VI simultaneously in several absorption systems (1.385, 1.674, 1.697) with the result that significant velocity 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 tentatively conclude that H I, O VI, and C IV are not formed in the same volumes and that therefore conclusions on ionization mechanisms are not possible from the observed column density ratios O VI/H I or O VI/C IV. The number density of O VI absorbers with W-rest greater than or equal to 25 m Angstrom 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 uncertain and further lines of sight will be probed in the next HST cycle. An estimate 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 ionization 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 factor of 15 between (z) over bar = 1.5 (this work) and the value found by Tripp 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.