Cosmic origins spectrograph observations of warm intervening gas at z ∼ 0.325 toward 3C 263

Abstract

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 6-8kms-1. The O VI λλ1031, 1037 lines are kinematically offset by Δv 12kms-1 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 × 105K. 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] -0.12dex, and a total hydrogen column density of N( H) 3 × 1019cm-2, which is 2dex 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 10 5-106K corresponding to the warm phase of the warm-hot intergalactic medium or shock-heated gas in the extended halos of galaxies. © 2012. The American Astronomical Society. All rights reserved..