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 \leq z _ { abs } \leq 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 _ { { H \textsc { i } } } = 13.9 and strong O vi ( N ( O vi ) /N ( H i ) \approx 1 ) and C iv doublets , while a nearby much stronger Ly \alpha absorber ( log N _ { { H \textsc { i } } } = 14.8 , \Delta v = 123 { km s ^ { -1 } } ) does not reveal any heavy element absorption .
For the first time high resolution observations allow 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 implications on ionization mechanisms are not possible from observed column density ratios O vi /H i or O vi /C iv .
The number density of O vi absorbers with W _ { rest } \geq 25 mà  is { d } N / { d } z \leq 10 , roughly a factor of 5 less than what has been found by Tripp at 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 \textsc { vi } } ) \approx 0.0003 h ^ { -1 } _ { 75 } 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 \bar { z } = 1.5 ( this work ) and the value found by Tripp et al. ( 2000 ) at \bar { z } = 0.21 , if the same oxygen abundance [ O / H ] = -1 is assumed .
Agreement with the simulations by Davé et al. ( 2001 ) can be obtained , if the oxygen abundance increases by a factor of \sim 3 over the same redshift interval .