Journal Article

Absorption signatures of warm-hot gas at low redshift: O <span class="smallCaps">vi</span>

Thorsten Tepper-García, Philipp Richter, Joop Schaye, C. M. Booth, Claudio Dalla Vecchia, Tom Theuns and Robert P. C. Wiersma

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 413, issue 1, pages 190-212
Published in print May 2011 | ISSN: 0035-8711
Published online April 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.18123.x
Absorption signatures of warm-hot gas at low redshift: O vi

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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 , Doppler parameter , 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 (10−1Z/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.

Keywords: methods: numerical; galaxies: formation; intergalactic medium; quasars: absorption lines; cosmology: theory

Journal Article.  19609 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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