Revealing the Warm-Hot Intergalactic Medium with O VI 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 of ~105-107 K. The O VI λλ1032, 1038 absorption line doublet in the far-UV portion of quasar spectra provides an important probe of this gas. Utilizing recent hydrodynamic simulations, it is found that there should be about five O VI absorption lines per unit redshift with equivalent widths of ≥35 mA, decreasing rapidly to ~0.5 per unit redshift at ≥350 mA. 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 mA; the remaining WHIM gas may be too hot or have too low a 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☉.