High-resolution O VI absorption line observations at 1.2 <= z <= 1.7 in the bright QSO HE 0515-4414

Abstract

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 <= 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 log N_ion HI = 13.9 and strong O Vi (N(O Vi)/N(H I) ~ 1) and C Iv doublets, while a nearby much stronger Ly alpha absorber (log N_ion HI = 14.8, Delta v = 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 >= 25 mÅ is dN/dz <= 10, roughly a factor of 5 less than that found by Tripp et al. (\cite{tri2:}) 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 (ion {O}{vi}) ~ 0.0003 h^-1₇₅ 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. (\cite{tri2:}) at bar {z} = 0.21, if the same oxygen abundance [O/H] = -1 is assumed. Agreement with the simulations by Davé et al. (\cite{dav:}) can be obtained, if the oxygen abundance increases by a factor of ~ 3 over the same redshift interval. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Aura, Inc., under NASA contract NAS 5-26555; and on observations collected at the VLT/Kueyen telescope, ESO, Paranal, Chile.