  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.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies