A Multiphase Absorber Containing O Vi and Broad H I Directly Tracing 106k 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 Å with a resolution of ∼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 λλ1216 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 Lyα 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 Lyα 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.