2010ApJ...719.1526S

High signal-to-noise observations of the QSO PKS 0405-123 (z_em= 0.572) with the Cosmic Origins Spectrograph from 1134 to 1796 Å with a resolution of ∼17 km/s are used to study the multi-phase partial Lyman limit system (LLS) at z = 0.16716, which has previously been studied using relatively low signal-to-noise spectra from STIS and FUSE. The LLS and an associated H I-free broad O VI absorber likely originate in the circumgalactic gas associated with a pair of galaxies at z = 0.1688 and 0.1670 with impact parameters of 116 h^–1₇₀ and 99 h^–1₇₀. The broad and symmetric O VI absorption is detected in the z = 0.16716 rest frame with v = -278±3 km/s, log N(O VI) = 13.90±0.03, and b = 52±2 km/s. This absorber is not detected in H I or other species with the possible exception of N V. The broad, symmetric O VI profile and the absence of corresponding H I absorption indicate that the circumgalactic gas in which the collisionally ionized O VI arises is hot (log T ∼ 5.8-6.2). The absorber may represent a rare but important new class of low-z intergalactic medium absorbers. The LLS has strong asymmetrical O VI absorption with log N(O VI) = 14.72±0.02 spanning a velocity range from -200 to +100 km/s. The high and low ions in the LLS have properties resembling those found for Galactic highly ionized high-velocity clouds where the O VI is likely produced in the conductive and turbulent interfaces between cool and hot gas.