Cosmic Origins Spectrograph Observations of Warm Intervening Gas Towards
  3C263
  We present HST/COS high S/N observations of the z = 0.32566 multi-phase
absorber towards 3C263. The COS data shows absorption from H I, O VI, C III, N
III, Si III and C II. The Ne VIII in this absorber is detected in the FUSE
spectrum. The low and intermediate ions are kinematically aligned with each
other and H I and display narrow line widths of 6 km/s. The O VI lines are
kinematically offset by 12 km/s from the low ions and are a factor of four
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 carbon abundance
in this gas phase is near-solar. The O VI and Ne VIII favor an origin in
collisionally ionized gas at T = 5.2 x 10^5 K. The H I absorption associated
with this warm absorber is a BLA marginally detected in the COS spectrum. This
warm gas phase has total hydrogen column density of N(H) ~ 3 x 10^19 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
temperature in the range of 10^5 - 10^6 K corresponding to the warm phase of
the WHIM or shock-heated gas in the extended halos of galaxies.
