Far Ultraviolet Spectroscopic Explorerand Space Telescope Imaging Spectrograph Observations of Intervening O vi Absorption Line Systems in the Spectrum of PG 0953+415

We present Far Ultraviolet Spectroscopic Explorer ( FUSE ) and Space Telescope Imaging Spectrograph (STIS) observations of the intergalactic medium toward the bright QSO PG 0953+415 ( z em = 0.239). The FUSE spectra extend from 905 to 1187 Å and have a resolution of 25 km s -1 , while the STIS spectra cover 1150-1730 Å and have a resolution of 7 km s -1 . Additional STIS observations at 30 km s -1 are obtained in selected wavelength ranges. An O VI system at z = 0.06807 is detected in H I Lyα, Lyβ, Lyγ, O VI λλ1031.93, 1037.62, N V λλ1238.82, 1242.80, C IV λλ1548.20, 1550.77, and C III λ977.02. The observed column densities can be modeled as a low-density intervening gas with a metallicity of 0.4 times solar in photoionization equilibrium with the ionizing extragalactic background radiation. The best fit is achieved with an ionization parameter, log U = -1.35, which implies n H ~ 10 -5 cm -3 and a path length of ~80 kpc through the absorbing gas. H I Lyα absorption at z = 0.14232 spans a velocity range of 410 km s -1 with the strongest components near 0 and 80 km s -1 in the z = 0.14232 rest frame. In this system, O VI λλ1031.93, 1037.62 absorption is strong near 0 km s -1 and not detected at 80 km s -1 . C III λ977.02 absorption is marginally detected at 80 km s -1 but is not detected at 0 km s -1 . The observations place constraints on the properties of the z = 0.14232 system but do not discriminate between collisional ionization in hot gas versus photoionization in a very low density medium with an ionization parameter log U > -0.74. The z = 0.06807 and 0.14232 O VI systems occur at redshifts where there are peaks in the number density of intervening galaxies along the line of sight determined from WIYN redshift measurements of galaxies in the ~1° field centered on PG 0953+415. We combine our observations of PG 0953+415 with those for other QSOs to update the estimate of the low-redshift number density of intervening O VI systems. Over a total unobscured redshift path of Δ z = 0.43, we detect six O VI systems with rest-frame equivalent widths of the O VI λ1031.93 line exceeding 50 mÅ, yielding dN / dz = 14 for ⟨ z ⟩ = 0.09. This implies a low-redshift value of the baryonic contribution to the closure density of the O VI systems of Ω b (O VI) > 0.002 h , assuming that the average metallicity in the O VI systems is 0.1 solar.