  High-resolution UV spectroscopy of low-z QSOs with the Space Telescope
Imaging Spectrograph and the Far Ultraviolet Spectroscopic Explorer has
indicated that O VI absorption-line systems provide a valuable probe of the
low-z intergalactic medium. These observations and their implications are
briefly reviewed. Though still uncertain due to the small sample, the number of
O VI absorbers per unit redshift is quite high, and these absorbers appear to
trace an important baryon reservoir. The O VI systems are intervening; they are
highly displaced from the background QSO redshifts and are correlated with
foreground luminous galaxies. Their physical conditions are variable and
sometimes complicated. In some cases, there is clear evidence that the
absorbers have a multiphase nature. Some appear to be photoionized by the UV
background from QSOs, while others are probably collisionally ionized. However,
the majority of the O VI lines have b-values that suggest an origin in
collisionally ionized hot gas. The observations are in agreement with
hydrodynamic simulations of cosmological structure growth, which predict
substantial quantities of shock-heated, hot intergalactic gas at low z, but
more observational and theoretical work is needed to confirm the nature of the
O VI systems.
