2001A&A...374..871R

STIS Echelle observations at a resolution of 10km/s and UVES/VLT spectroscopy at a resolution of 7km/s of the luminous QSO HE 0515-4414 (z_em=1.73, B=15.0) reveal four intervening OVI absorption systems in the redshift range 1.21≤z_abs≤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 logN_HI=13.9 and strong OVI (N(OVI)/N(HI)≃1) and CIV doublets, while a nearby much stronger Ly α absorber (logN_HI=14.8, Δv=123km/s) does not reveal any heavy element absorption. For the first time, high resolution observations allow one to measure radial velocities of HI, CIV and OVI simultaneously in several absorption systems (1.385, 1.674, 1.697) with the result that significant velocity differences (up to 18km/s, are observed between HI and OVI, while smaller differences (up to 5km/s) are seen between CIV and OVI. We tentatively conclude that HI, OVI, and CIV are not formed in the same volumes and that therefore conclusions on ionization mechanisms are not possible from the observed column density ratios OVI/HI or OVI/CIV. The number density of OVI absorbers with W_rest≥25mÅ is dN/dz≤10, roughly a factor of 5 less than that found by Tripp et al. (2000ApJ...542...42T) 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 OVI-phase yields Ω_b(OVI)≃0.0003h^–1₇₅ for [O/H]=-1 and an assumed ionization fraction OVI/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. (2000ApJ...542...42T) at z=0.21, if the same oxygen abundance [O/H]=-1 is assumed. Agreement with the simulations by Dave et al. (2001ApJ...552..473D) can be obtained, if the oxygen abundance increases by a factor of ∼3 over the same redshift interval.