Low Redshift Intergalactic Absorption Lines in the Spectrum of He 0226–4110

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Space Telescope Imaging Spectrograph (STIS E140M) spectra of HE0226–4110 (zem = 0.495) that have a nearly continuous wavelength coverage from 910 to 1730 Å. We detect 56 Lyman absorbers and 5 O VI absorbers. The number of intervening O VI systems per unit redshift with Wλ & 50 mÅ is dN (O VI)/dz ≈ 11. For 4 of the 5 O VI systems other ions (such as C III, C IV, O III, O IV) are detected. The O VI systems unambiguously trace hot gas only in one case. For the 4 other O VI systems, photoionization and collisional ionization models are viable options to explain the observed column densities of the O VI and the other ions. If photoionization applies for those systems, the broadening of the metal lines must be mostly non-thermal or several components may be hidden in the noise, but the H I broadening appears to be mostly thermal. If the O VI systems are mostly photoionized, only a fraction of the observed O VI will contribute to the baryonic density of the warm-hot ionized medium (WHIM) along this line of sight. Combining our results with previous ones, we show that there is a general increase of N(O VI) with increasing b(O VI). Cooling flow models can reproduce the N–b distribution but fail to reproduce the observed ionic ratios. A comparison of the number of O I, O II, O III, O IV, and O VI systems per unit redshift show that the low-z IGM is more highly ionized than weakly ionized. We confirm that photoionized O VI systems show a decreasing ionization parameter with increasing H I column density. O VI absorbers with collisional ionization/photoionization degeneracy follow this relation, possibly suggesting that they are principally photoionized. We find that the photoionized O VI systems in the low redshift IGM have a median abundance of 0.3 solar. We do not find additional Ne VIII systems other than the one found by Savage et al., although our sensitivity should have allowed the detection of Ne VIII in O VI systems at T ∼ (0.6 − 1.3) × 10 K (if collisional ionization equilibrium applies). Since the bulk of the WHIM is believed to be at temperatures T > 10 K, the hot part of the WHIM remains to be discovered with FUV–EUV metal-line transitions. Subject headings: cosmology: observations — intergalactic medium — quasars: absorption lines — quasars: individual (HE 0226–4110) Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555. Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706 Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218. Department of Astronomy, University of Massachusetts, Amherst, MA 01003.