Low-redshift intergalactic absorption lines in the spectrum of HE 0226-4110

We present an analysis of the FUSE and HST STIS E140M spectra of HE 0226-4110 (z = 495). We detect 56 Lyman absorbers and five O VI absorbers. The number of intervening O VI systems per unit redshift with W-gimel greater than or similar to 50 m angstrom is dN(O VI)/dz approximate to 11. The O VI systems unambiguously trace hot gas only in one case. For the four other O VI systems, photoionization and collisional ionization models are viable options to explain the observed column densities of O VI and the other ions. 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). A comparison of the number of O I, O II, O III, O IV, and O VI systems per unit redshift shows 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 similar to (0.6 - 3); 10(6) K (if CIE applies). The hot part of the WHIM with T > 10(6) K is believed to contain most of the baryons. However, this hot phase of the WHIM remains to be discovered with FUV-EUV metal-line transitions.