FUSE and STIS observations of the warm-hot intergalactic medium toward PG 1259+593

We use Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) spectra to study intergalactic absorption toward the quasar PG 1259+593 (z(em) = 0: 478), with a particular emphasis on the warm-hot intergalactic medium (WHIM). The combined FUSE and STIS spectrum of PG 1259+593 covers the full wavelength range between 905 and 1730 Angstrom at a spectral resolution of similar to25 km s(-1) for the FUSE bandpass (lambda less than or equal to 1180 Angstrom) and similar to7 km s(-1) for the STIS range (lambda > 1150 Angstrom). The signal-to-noise ratios (S/N) per resolution element are similar to 10 - 30 (FUSE) and similar to 7 - 17 ( STIS). We identify 135 intergalactic absorption lines with equivalent widths greater than or equal to10 mAngstrom, tracing 78 absorption components in 72 Lyalpha/Lybeta absorption-line systems. Metal-line absorption by species such as C III, C IV, O III, O IV, O vi, and Si III is clearly detected in four systems and is possibly seen in four additional cases. We study the distribution and physical properties of the WHIM as sampled by O vi and intrinsically broad Lyalpha lines. The number of intervening O vi absorbers for equivalent widths W-tau greater than or equal to 24 mAngstrom is 3 - 6 over an unobscured redshift path of Deltaz approximate to 0.368. This implies a number density of O vi systems, dN(O IV)/dz, of similar to 8 - 16 above this equivalent width limit along this sight line. A seventh intervening O vi absorber is possibly detected with W-r(O VI) approximate to 15 mAngstrom. The range of dN(O IV)/dz = 8-16 for W-tau greater than or equal to 24 mAngstrom is consistent with estimates from other sight lines, supporting the idea that intervening intergalactic O vi absorbers contain an substantial fraction of the baryonic mass in the low-redshift universe. We identify a number of broad Lyalpha absorbers with large Doppler parameters ( b similar to 40-200 km s(-1)) and low column densities [N( H I) < 10(14) cm(-2)]. For pure thermal broadening, these widths correspond to temperatures of ∼1 x 10(5) - 3 x 10(6) K. While these broad absorbers could be caused by blends of multiple, unresolved lines, continuum undulations, or by kinematic flows and Hubble broadening, we consider the possibility that some of these features are single-component, thermally broadened Lyα lines. These systems could represent WHIM absorbers that are too weak, too metal-poor, and/or too hot to be detected in O vi. If so, their widths and their frequency in the PG 1259+593 spectrum imply that these absorbers trace an even larger fraction of the baryons in the low-redshift universe than the O vi absorbing systems. A thermal Doppler broadening explanation for one of these broad features is supported by the probable detection of O VI near the velocity of a broad Lyα and Lyβ absorber with an O vi line width ∼4 times smaller than for H I.