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

Abstract

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(OVI)=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 logN(NeVIII)=13.89+/-0.11, b=23+/-15 km s^-1, and v=-7+/-6 km s^-1 and logN(OVI)=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(NeVIII)/N(OVI)=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(NeVIII)/N(OVI)=0.33+/-0.10 is consistent with gas in collisional ionization equilibrium near T=5.4×10⁵ K with logN(H)=19.9 and N(H)/N(HI)=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, logU~-1.85, T~2.1×10⁴ K, n_H~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.

Based partly on observations with (1) 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 NAS5-26555, and (2) the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer operated by Johns Hopkins University, supported by NASA contract NAS5-32985.