Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph observations of intervening OVI 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 Angstrom and have a resolution of 25 km s(-1), while the STIS spectra cover 1150-1730 Angstrom 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 Lyalpha, Lybeta, Lygamma, O VI lambdalambda1031.93, 1037.62, N V lambdalambda1238.82, 1242.80, C IV lambdalambda1548.20, 1550.77, and C III lambda977.02. The observed column densities can be modeled as a low-density intervening gas with a metallicity of 0.4(-0.2)(+0.6) 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) similar to 10(-5) cm(-3) and a path length of similar to80 kpc through the absorbing gas. H I Lyalpha 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 lambdalambda1031.93, 1037.62 absorption is strong near 0 km s(-1) and not detected at 80 km s(-1). C III lambda977.02 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 similar to1degrees 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 Deltaz = 0.43, we detect six O VI systems with rest-frame equivalent widths of the O VI lambda1031.93 line exceeding 50 mAngstrom, yielding dN/dz = 14(-6)(+9) for [z] = 0.09. This implies a low-redshift value of the baryonic contribution to the closure density of the O VI systems of Omega(b)(O VI) > 0.002 (-1)(h75), assuming that the average metallicity in the O VI systems is 0.1 solar.