Detection of Ne VIII in the Low-Redshift Warm-Hot Intergalactic Medium*

摘要

High-resolution FUSE and STIS observations of the bright QSO HE 0226-4110 (zem = 0.495) reveal the presence of a multiphase absorption-line system at zabs(O ) = 0.20701 containing absorption from H I (Lyα to Lyθ), 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 ) = 13.89 ± 0.11, b = 23 ± 15 km s-1, and v = -7 ± 6 km s-1 and log N(O ) = 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 σ significance levels, respectively. This represents the first detection of intergalactic Ne VIII, a diagnostic of gas with temperature in the range from ~5 × 105 to ~1 × 106 K. Through the entire absorber, N(Ne )/N(O ) = 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 ~11 Mpc (assuming [Z/H] = -0.5) implies that the Hubble flow absorption-line broadening would be ~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 )/N(O ) = 0.33 ± 0.10 is consistent with gas in collisional ionization equilibrium near T = 5.4 × 105 K with log N(H) = 19.9 and N(H)/N(H ) = 1.7 × 106. Various nonequilibrium ionization processes are also considered, because gas with T ~ (1-6) × 105 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 ~ -1.85, T ~ 2.1 × 104 K, nH ~ 2.6 × 10-5 cm-3, P/k ~ 0.5 cm-3 K, and a path length of ~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.