High-resolution OVI absorption line observations at 1.2 $ \leq z \leq $ 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 (zem =1 :73, B =1 5:0) reveal four intervening O vi absorption systems in the redshift range 1:21 zabs 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 NH i = 13:9 and strong O vi (N(O vi)/N(H i) 1) and C iv doublets, while a nearby much stronger Ly absorber (log NH i =1 4:8, v = 123 km s 1 ) does not reveal any heavy element absorption. For the rst 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 signicant velocity dierences (up to 18 km s 1 ,a re observed between H i and O vi, while smaller dierences (up to 5 km s 1 ) are seen between C iv and O vi .W e tentatively conclude that H i ,O vi ,a nd Civ 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 .T he number density of O vi absorbers with Wrest 25 m Ai s dN=dz 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 b(O vi) 0:0003h 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 z =1 :5 (this work) and the value found by Tripp et al. (2000) at z =0 :21, if the same oxygen abundance (O=H) = 1 is assumed. Agreement with the simulations by Dav e et al. (2001) can be obtained, if the oxygen abundance increases by a factor of3 over the same redshift interval.

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