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| Paper: |
Discovery of a Hot, Multiphase Ne VIII/O VI Absorber in the Low-Redshift IGM |
| Volume: |
348, Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE |
| Page: |
363 |
| Authors: |
Savage, B.D.; Lehner, N.; Wakker, B.P.; Sembach, K.R.; Tripp, T.M. |
| Abstract: |
High resolution FUSE and STIS observations of the bright QSO HE0226–4110 (zem = 0.495) reveal the presence of a multi-phase absorption line system at zabs(O VI) = 0.20701 containing absorption from H I (Ly α to Ly θ), C III, O III, O IV, O VI, N III, Ne VIII, Si III, and S VI. This represents the first detection of intergalactic Ne VIII, a diagnostic of gas with T ∼ 7×105 K. Single component fits to the Ne VIII and O VI absorption yield logN(Ne VIII) = 13.89 ± 0.11, v = −7 ± 6 kms−1 and logN(O VI) = 14.37 ± 0.03, v = 0 ± 2 kms−1. Through the entire absorber, N(Ne VIII)/N(O VI) = 0.33 ± 0.10. The O VI and Ne VIII are not likely created in a low density medium photoionized solely by the extragalactic background at z = 0.2; the required path length for [Z/H] = −0.5 of ∼11 Mpc implies the Hubble flow absorption line broadening would be ∼10 times greater than the observed line widths. With [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 × 105 K with logN(H) = 19.9 and N(H)/N(H I) = 1.7 × 106. 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, T = 2.1 × 104 K, nH = 2.6 × 10−5 cm−3, P/k ∼ 0.5 cm−3 K, and path length of ∼60 kpc. These values suggest 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. |
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