Highly Ionized Plasma in the Halo of a Luminous Spiral Galaxy Near
  z=0.225
  We present analyses of the physical conditions in the z=0.22496 and z=0.22638
multi-phase O VI absorption systems detected in the ultraviolet HST/STIS and
FUSE spectra of the quasar H1821+643. Both absorbers are likely associated with
the extended halo of a ~2L* Sbc-Sc galaxy situated at a projected distance of
~116 kpc from the sight line. The z=0.22496 absorber is detected in C II, C
III, C IV, O III, O VI, Si II, Si III and H I at > 3 sigma significance. The
low and intermediate ions in this absorber are consistent with an origin in
photoionized gas with [Si/H] and [C/H] of -0.6 dex. In contrast, the broader O
VI absorption is likely produced in collisionally ionized plasma under
nonequilibrium conditions. The z=0.22638 system has broad-Lya (BLA) and C III
absorption offset by v = -53 km/s from O VI. The H I and C III line widths for
the BLA imply T = 1.1 x 10^5 K. For non-equilibrium cooling we obtain [C/H] of
-1.5 dex and a total hydrogen column of N(H) = 3.2 x 10^{18} cm^-2 in the BLA.
The O VI, offset from the BLA with no detected H I or C III, is likely
collisionally ionized at T ~ 3 x 10^5 K. From the observed multiphase
properties and the proximity to a luminous galaxy, we propose that the
z=0.22496 absorber is an extragalactic analog of a highly ionized Galactic HVC,
in which the O VI is produced in transition temperature plasma (T ~ 10^5 K) at
the interface layers between the warm HVC gas phase and the hot coronal halo of
the galaxy. The z=0.22638 O VI - BLA absorber could be tracing a cooling
condensing fragment in the nearby galaxy's hot gaseous halo.
