Highly-Ionized Intergalactic Gas at Low Redshifts: Constraints from QSO Absorption Lines

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.