Detection of NeVIII in the low-redshift warm-hot intergalactic medium

High-resolution FUSE and STIS observations of the bright QSO HE 0226-4110 (z(em) = 0: 495) reveal the presence of a multiphase absorption-line system at z(abs)(O VI) = 0.20701 containing absorption from H I (Ly alpha to Ly theta), C III, O III, O IV, O VI, N III, Ne VIII, Si III, S VI, and possibly S V. Single-component fits to the Ne VIII and O VI absorption doublets yield log N(Ne VIII) = 13.89 +/- 0.11, b = 23 +/- 15 km s(-1), and v = -7 +/- 6 km s(-1) and log N(O VI) = 14.37 +/- 0: 03, b = 31 +/- 2 km s(-1), and v = 0 +/- 2 km s(-1). The Ne VIII and O VI doublets are detected at 3.9 and 16 sigma significance levels, respectively. This represents the first detection of intergalactic Ne VIII, a diagnostic of gas with temperature in the range from similar to 5 x 10(5) to -1 x 10(6) K. Through the entire absorber, N(Ne VIII)/N(O VI) = 0.33 +/- 0: 10. The O VI and Ne VIII are not likely to have been created in a low-density medium photoionized solely by the extragalactic background at z = 0: 2, since the required path length similar to 11 Mpc ( assuming [Z/H] = -0.5) implies that the Hubble flow absorption-line broadening would be similar to 10 times greater than the observed line widths. A collisional ionization origin is therefore more likely. Assuming [Ne/H] and [O/H] = -0.5, the value N(Ne VIII)/N(O VI) = 0.33 +/- 0.10 is consistent with gas in collisional ionization equilibrium near T = 5.4 x 10(5) K with log N(H) = 19.9 and N(H)/N(H I) =1.7 10(6). Various nonequilibrium ionization processes are also considered, because gas with T similar to (1-6) x 10(5) K cools efficiently. The observations of O VI and Ne VIII in the z = 0.20701 system support the basic idea that a substantial fraction of the baryonic matter at low redshift exists in hot, very highly ionized gaseous structures. Absorption by the moderately ionized gas ( including O IV and S VI) is well modeled by gas in photoionization equilibrium with [Z/H] = -0.5 +/- 0.2, log U similar to -1.85, T similar to 2.1 x 10(4) K, n(H) similar to 2.6 x 10(-5) cm(-3), P/k similar to 0.5 cm(-3) K, and a path length of similar to 60 kpc. These values suggest that the moderately ionized absorber may be associated with the modestly enriched photoionized gas in a galaxy group or the outermost regions of a galaxy halo.