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 (W HIM) with temperatures of similar to 10(5)-10(7) K. The O VI lambda lambda 1032, 1038 absorption line doublet in the far-UV portion of quasar spectra provides an important probe of this gas. Utilizing recent hydrodynamic simu lations, it is found that there should be about five O VI absorption lines per unit redshift with equivalent widths of greater than or equal to 35 m A ngstrom, decreasing rapidly to similar to0.5 per unit redshift at greater t han or equal to 350 m Angstrom. About 10% of the total baryonic matter or 2 0%-30% of the WHIM is expected to be in the O VI absorption line systems wi th equivalent width greater than or equal to 20 m Angstrom; the remaining W HIM 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 reg ard to the line abundance and total mass contained in these systems. Some o f 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 somew hat connected network of filaments. The typical metallicity of these region s is 0.1-0.3 Z..