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 A with a resolution of ~17 km/s and signal-to- noise per resolution element of 20 to 40 are used to study a multi-phase partial Lyman limit system (LLS) at z = 0.22601 tracing both cool and hot gas. FUSE 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 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 = -2.8pm0.1 and log N(H) = 19.35pm0.18, log n(H) = -2.9pm0.2, log T = 4.27pm0.02, log (P/k) = 1.75pm0.17, and log L(kpc) = 0.70pm0.25. 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 multi-phase system has strong O VI and associated broad Ly {alpha} absorption (BLA) with log N(O VI) = 14.21pm0.02, b(O VI) = 37pm1 km/s, log N(H I) = 13.70(+0.05,-0.08), b(H I)=140 (+14, -16) km/s and b(H I)/b(O VI) = 3.9pm0.4. 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.

O VI Absorbers Tracing Hot Gas Associated with a Pair of Galaxies at z = 0.167

High signal-to-noise (S/N) observations of the QSO PKS 0405-123 (zem = 0.572) with the Cosmic Origins Spectrograph from 1134 to 1796 A with a resolution of 17 km s-1 are used to study the multi-phase partial Lyman limit system (LLS) at z = 0.16716 which has previously been studied using relatively low S/N 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 h70-1 and 99 h70-1. The broad and symmetric O VI absorption is detected in the z = 0.16716 restframe with v = -278 +/- 3 km s-1, log N(O VI) = 13.90 +/- 0.03 and b = 52 +/- 2 km s-1. 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 absence of corresponding H I absorption indicates 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 IGM 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-1. The high and low ions in the LLS have properties resembling those found for Galactic highly ionized HVCs where the O VI is likely produced in the conductive and turbulent interfaces between cool and hot gas.