Revealing the Warm-hot Intergalactic Medium with Ovi Absorption

Hydrodynamic simulations of growth of cosmic structure suggest that 30-50% of the total baryons at z = 0 may be in a warm-hot intergalactic medium (WHIM) with temperatures ∼ 10 − 10K. The O VI λλ1032, 1038 absorption line doublet in the FUV portion of QSO spectra provides an important probe of this gas. Utilizing recent hydrodynamic simulations, it is found that there should be ∼ 5 O VI absorption lines per unit redshift with equivalent widths ≥ 35 mÅ, decreasing rapidly to ∼ 0.5 per unit redshift at ≥ 350 mÅ. About 10% of the total baryonic matter or 20− 30% of the WHIM is expected to be in the O VI absorption line systems with equivalent width ≥ 20 mÅ; the remaining WHIM gas may be too hot or have too low metallicity to be detected in O VI. We find that the simulation results agree well with observations with regard to the line abundance and total mass contained in these systems. Some of the O VI systems are collisionally ionized and some are photoionized, but most of the mass is in the collisionally ionized systems. We show that the gas that produces the O VI absorption lines does not reside in virialized regions such as galaxies, groups, or clusters of galaxies, but rather has an overdensity of 10 − 40 times the average density. These regions form a somewhat connected network of filaments. The typical metallicity of these regions is 0.1− 0.3 Z⊙. Subject headings: Cosmology: large-scale structure of Universe – cosmology: theory – intergalactic medium – quasars: absorption lines – numerical method