Revealing the Warm-Hot Intergalactic Medium with O [CSC]vi[/CSC] Absorption

Abstract

Hydrodynamic simulations of growth of cosmic structure suggest that 30%-50% of the total baryons at may be in a warm-hot intergalactic medium (WHIM) with temperatures of ∼10 5-10 7 K. The O vi ll1032, z p 0 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 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 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 ,