2010ApJ...712.1443N

We present analyses of the physical conditions in the {z(O VI) = 0.22496} and {z(O VI) = 0.22638} multiphase absorption systems detected in the ultraviolet Hubble Space Telescope/STIS and FUSE spectra of the quasar H 1821+643 (m_V= 14.2, z_em= 0.297). Both absorbers are likely associated with the extended halo of a ∼2L*_BSbc-Sc galaxy situated at a projected distance of ∼116 h^–1₇₁ 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 ≲ 3x10⁴ 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 VI) = 0.22638} system has broad Ly α (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 = 1.1x10⁵ K. For non-equilibrium cooling we obtain [C/H] ~-1.5 dex and N(H) = 3.2x10¹⁸/cm2 in the BLA. The O VI, offset from the BLA with no detected H I or C III, is likely collisionally ionized at T ∼ 3x10⁵ 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 < 5x10⁴ 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.