2004ApJS..153..165R

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 Å at a spectral resolution of ∼25 km/s for the FUSE bandpass (λ≤1180 Å) and ∼7 km/s for the STIS range (λ>1150 Å). The signal-to-noise ratios (S/N) per resolution element are ∼10-30 (FUSE) and ∼7-17 (STIS). We identify 135 intergalactic absorption lines with equivalent widths ≥10 mÅ, tracing 78 absorption components in 72 Lyα/Lyβ 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 Lyα lines. The number of intervening O VI absorbers for equivalent widths W_τ≥24 mÅ is 3-6 over an unobscured redshift path of Δz~0.368. This implies a number density of O VI systems, dN_OIV_/dz, of ∼8-16 above this equivalent width limit along this sight line. A seventh intervening O VI absorber is possibly detected with W_r(OVI)~15 mÅ. The range of dN_OIV_/dz=8-16 for W_τ≥24 mÅ 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 Lyα absorbers with large Doppler parameters (b∼40-200 km/s) and low column densities [N(HI)<10¹⁴/cm2]. For pure thermal broadening, these widths correspond to temperatures of ∼1x10⁵-3x10⁶ 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.