A Multiphase Absorber Containing O VI and Broad H I Directly Tracing 10^6 K Plasma at Low-Redshift Toward HE 0153-4520

Observations of the QSO HE 0153-4520 (z(em) = 0.450) with the Cosmic Origins Spectrograph (COS) from 1134 to 1796 angstrom with a resolution of similar to 17 km s(-1) and a signal-to-noise ratio per resolution element of 20-40 are used to study a multiphase partial Lyman limit system (LLS) at z = 0.22601 tracing both cool and hot gas. Far-Ultraviolet Spectroscopic Explorer observations of the Lyman limit break yield log N(H I) = 16.61 (0.12, -0.17). The observed UV absorption lines of H I lambda lambda 1216 to 926, C III, C II, N III, N II, Si III, and Si II imply the existence of cool photoionized gas in the LLS with log U = -2.8 +/- 0.1 and log N(H) = 19.35 +/- 0.18, log n(H) = -2.9 +/- 0.2, log T = 4.27 +/- 0.02, log (P/k) = 1.75 +/- 0.17, and log L (kpc) = 0.70 +/- 0.25. The abundances are [X/H] = -0.8 (+0.3, -0.2) for N, Si, and C, but the result is sensitive to the assumed shape of the ionizing background radiation field. The multiphase system has strong O VI and associated broad Ly alpha absorption (BLA) with log N(O VI) = 14.21 +/- 0.02, b(O VI) = 37 +/- 1 km s(-1), log N(H I) = 13.70 (+0.05, -0.08), b(H I) = 140 (+14, -16) km s(-1) and b(H I)/b(O VI) = 3.9 +/- 0.4. The O VI does not arise in the cool photoionized gas of the LLS. The O VI and BLA imply the direct detection of thermally broadened absorption by hot gas with log T = 6.07 (+0.09, -0.12), [O/H] = -0.28 (+0.09, -0.08), and log N(H) = 20.41 (+0.13, -0.17). The absorber probably occurs in the circumgalactic environment (halo) of a foreground galaxy. The very high sensitivity of UV absorption line spectroscopy to O VI and thermally broadened H I Ly alpha now permits the direct detection of hot gas with log T > 6, a determination of the abundance of O/H, and log N(H) in low-redshift intergalactic medium absorption line systems.