Probing the intergalactic medium with the OVI forest

Recent Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer observations have detected O VI absorption lines at low redshift that are not clearly associated with any galactic system. In this Letter, we argue that these lines are due to metal-enriched hot gas in the intergalactic medium. Using numerical simulations of a cosmological constant-dominated cosmology, combined with reasonable assumptions about the metallicity distribution, we show that the number density and internal characteristics of these lines are correctly predicted. We find that the O VI is primarily produced by collisional ionization from gas at a few times 10(5) K for lines with equivalent widths larger than 40 m Angstrom, while weaker lines can also be produced by photoionization. The absorption occurs in diffuse gas in filaments at moderate overdensity (delta similar to 5-100).