We investigate the origin and physical properties of O vi absorbers at low redshift ( z = 0.25 ) using a subset of cosmological , hydrodynamical simulations from the OverWhelmingly Large Simulations ( OWLS ) project .
Intervening O vi 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 O vi line parameters ( column density { N _ { \hbox { \scriptsize O { \tiny VI } } } } , Doppler parameter b _ { \hbox { \scriptsize O { \tiny VI } } } , rest equivalent width W _ { r } ) from our simulations exhibit a lack of strong O vi absorbers , a discrepancy that has also been found by .
This suggests that physical processes on sub-grid scales ( e.g .
turbulence ) may strongly influence the observed properties of O vi systems .
We find that the intervening O vi absorption arises mainly in highly metal-enriched ( 10 ^ { -1 } \ll Z / Z _ { \odot } \lesssim 1 ) gas at typical overdensities of 1 \ll \rho / \left < \rho \right > \lesssim 10 ^ { 2 } .
One third of the O vi absorbers in our simulation are found to trace gas at temperatures T < 10 ^ { 5 } \leavevmode \nobreak { K } , while the rest arises in gas at higher temperatures , most of them around T = 10 ^ { 5.3 \pm 0.5 } { K } .
These temperatures are much higher than inferred by , probably because that work did not take the suppression of metal-line cooling by the photo-ionising background radiation into account .
While the O vi resides in a similar region of ( \rho,T ) -space as much of the shock-heated baryonic matter , the vast majority of this gas has a lower metal content and does not give rise to detectable O vi absorption .
As a consequence of the patchy metal distribution , O vi 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 mixing with the ambient gas and cooling .
The common approach of comparing O vi and H i column densities to estimate the physical conditions in intervening absorbers from QSO observations may be misleading , as most of the H i ( and most of the gas mass ) is not physically connected with the high-metallicity patches that give rise to the O vi absorption .