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Title
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Absorption signatures of warm-hot gas at low redshift : O vi
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Author
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Abstract
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| 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 inline image, Doppler parameter inline image, rest equivalent width Wr) from our simulations exhibit a lack of strong O vi absorbers, a discrepancy that has also been found by Oppenheimer & Davé. This suggests that physical processes on subgrid 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≪Z/Z⊙≲ 1) gas at typical overdensities of 1 ≪ρ/〈ρ〉≲ 102. One-third of the O vi absorbers in our simulation are found to trace gas at temperatures T < 105 K, while the rest arises in gas at higher temperatures, most of them around T= 105.3 ± 0.5 K. These temperatures are much higher than inferred by Oppenheimer & Davé, probably because that work did not take the suppression of metal-line cooling by the photoionizing background radiation into account. While the O vi resides in a similar region of (ρ, 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 per cent) 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. |
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Language
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English
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Source (journal)
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Monthly notices of the Royal Astronomical Society. - Oxford, 1983, currens
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Publication
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Oxford
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2011
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ISSN
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0035-8711
[print]
1365-2966
[online]
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DOI
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10.1111/J.1365-2966.2010.18123.X
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Volume/pages
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413
:1
(2011)
, p. 190-212
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ISI
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000289525000014
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Full text (Publisher's DOI)
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