2012ApJ...752...65N

We present HST/COS high-S/N observations of the z = 0.32566 multiphase absorber toward 3C 263. The Cosmic Origins Spectrograph (COS) data show absorption from H I (Lyα to Lyθ), O VI, C III, N III, Si III, and C II. The Ne VIII in this absorber is detected in the FUSE spectrum along with O III, O IV, and N IV. The low and intermediate ions are kinematically aligned with each other and H I and display narrow line widths of b ∼ 6-8 km/s. The O VI λλ1031, 1037 lines are kinematically offset by Δv ∼ 12 km/s from the low ions and are a factor of ∼4 broader. All metal ions except O VI and Ne VIII are consistent with an origin in gas photoionized by the extragalactic background radiation. The bulk of the observed H I is also traced by this photoionized medium. The metallicity in this gas phase is Z ≳ 0.15 Z_☉ with carbon having near-solar abundances. The O VI and Ne VIII favor an origin in collisionally ionized gas at T = 5.2 x10⁵ K. The H I absorption associated with this warm absorber is a broad-Lyα absorber (BLA) marginally detected in the COS spectrum. This warm gas phase has a metallicity of [X/H] ~-0.12 dex, and a total hydrogen column density of N( H) ∼ 3x10¹⁹/cm2, which is ∼2 dex higher than what is traced by the photoionized gas. Simultaneous detection of O VI, Ne VIII, and BLAs in an absorber can be a strong diagnostic of gas with T ∼ 10⁵-10⁶ K corresponding to the warm phase of the warm-hot intergalactic medium or shock-heated gas in the extended halos of galaxies.