| Abstract |
We present analyses of the physical conditions in the z(\O\,{\sc vi}) =
0.22496 and z(\O\,{\sc vi}) = 0.22638 multiphase absorption systems
detected in the ultraviolet Hubble Space Telescope/STIS and FUSE spectra
of the quasar H 1821+643 (mV = 14.2, zem = 0.297).
Both absorbers are likely associated with the extended halo of a
~2L* B Sbc-Sc galaxy situated at a projected distance of
~116 h -1 71 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 (Ly α-Lyθ) at >3σ significance.
The components of Si III and Si II are narrow with implied temperatures
of T <~ 3 × 104 K. The low and intermediate ions in
this absorber are consistent with an origin in a T ~ 104 K
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(\O\,{\sc vi}) = 0.22638
system has broad Ly α (BLA) and C III absorption offset by v =
-53 km s-1 from O VI. The H I and C III line
widths for the BLA imply T = 1.1 × 105 K. For
non-equilibrium cooling we obtain [C/H] ~-1.5 dex and N(H) = 3.2
× 1018 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 × 105 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 ~ 105 K) at the interface layers
between the warm (T < 5 × 104 K) HVC gas phase and
the hot (T >~ 106 K) 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.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS 05-26555, and the NASA-CNES/ESA Far Ultraviolet
Spectroscopic Explorer mission, operated by the Johns Hopkins
University, supported by NASA contract NAS 05-32985. |