Absorption signatures of warm-hot gas at low redshift: OVI
  We investigate the origin and physical properties of OVI absorbers at low
redshift (z = 0.25) using a subset of cosmological, hydrodynamical simulations
from the OverWhelmingly Large Simulations (OWLS) project. Intervening OVI
absorbers are believed to trace shock-heated gas in the Warm-Hot Intergalactic
Medium (WHIM) and may thus play a key role in the search for the missing
baryons in the present-day Universe. When compared to observations, the
predicted distributions of the different OVI line parameters (column density,
Doppler parameter, rest equivalent width) from our simulations exhibit a lack
of strong OVI absorbers. This suggests that physical processes on sub-grid
scales (e.g. turbulence) may strongly influence the observed properties of OVI
systems. We find that the intervening OVI absorption arises mainly in highly
metal-enriched (0.1 << Z/Z_sun < 1) gas at typical overdensities of 1 <<
rho/<rho> < 100. One third of the OVI absorbers in our simulation are found to
trace gas at temperatures T < 10^5 K, while the rest arises in gas at higher
temperatures around T =10^5.3 K. The OVI resides in a similar region of
(rho,T)-space as much of the shock-heated baryonic matter, but the vast
majority of this gas has a lower metal content and does not give rise to
detectable OVI absorption As a consequence of the patchy metal distribution,
OVI absorbers in our simulations trace only a very small fraction of the cosmic
baryons (<2 percent) and the cosmic metals. Instead, these systems presumably
trace previously shock-heated, metal-rich material from galactic winds that is
now cooling. The common approach of comparing OVI and HI column densities to
estimate the physical conditions in intervening absorbers from QSO observations
may be misleading, as most of the HI (and most of the gas mass) is not
physically connected with the high-metallicity patches that give rise to the
OVI absorption.
