2002ApJ...564..631S

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^+0.6_–0.2 times solar in photoionization equilibrium with the ionizing extragalactic background radiation. The best fit is achieved with an ionization parameter, logU=-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 logU>-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 Δz=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⁺⁹_–6 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(OVI)>0.002 h^–1₇₅, assuming that the average metallicity in the O VI systems is 0.1 solar.