Absorption signatures of warm-hot gas at low redshift: O vi

Thorsten Tepper-Garca 2 Philipp Richter 2 Joop Schaye 1 C. M. Booth 1 Claudio Dalla Vecchia 0 1 Tom Theuns 4 5 Robert P. C. Wiersma 1 3 0 Max Planck Institut fu r Extraterrestrische Physik , Giessenbachstrae 1, 85748 Garching , Germany 1 Leiden Observatory, Leiden University , PO Box 9513, 2300 RA Leiden , the Netherlands 2 Universita t Potsdam , Karl-Liebknecht-Str. 24/25, 14476 Potsdam , Germany 3 Max Planck Institut fu r Astrophysik , Karl-Schwarzschild-Str. 1, 8574 Garching , Germany 4 Department of Physics, University of Antwerp , Groenenborgerlaan 171, B-2020 Antwerpen , Belgium 5 Institute for Computational Cosmology, Department of Physics, University of Durham , South Road, Durham, DH1 3LE A B S T R A C T 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 NO VI, Doppler parameter bO VI, rest equivalent width Wr) from our simulations exhibit a lack of strong O VI absorbers, a discrepancy that has also been found by Oppenheimer & Dave. 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 (101 Z/Z 1) gas at typical overdensities of 1 / 102. Onethird 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 = 105.30.5 K. These temperatures are much higher than inferred by Oppenheimer & Dave, 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 (, 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. - Diffuse ionized gas in the intergalactic medium (IGM) represents the major baryon reservoir in the Universe at any redshift. From observations of the Ly forest line density at high redshift in quasar (QSO) and active galactic nuclei (AGN) spectra, it can be deduced that the photoionized IGM contains more than 90 per cent of the baryons at redshift z = 3 (Rauch, Haehnelt & Steinmetz 1997; Weinberg et al. 1997; Schaye 2001). However, at z = 0 the fraction of baryons residing in the Ly forest is strongly reduced to 3040 per cent (Penton, Stocke & Shull 2004), while at the same time the (observable) amount of baryons in condensed structures (i.e. baryons in galaxies and galaxy clusters) has increased to only a few per cent (Fukugita 2004). These observations thus indicate that a significant fraction of the baryons formerly residing in the photoionized IGM have disappeared. Cosmological simulations have predicted that, as a result of the large-scale structure formation in the Universe, most of these missing baryons have moved into a hot, shock-heated intergalactic gas phase (Cen & Ostriker 1999; Dave et al. 2001; Bertone, Schaye & Dolag 2008). This shock-heated intergalactic gas phase is referred to as the warm-hot intergalactic medium (WHIM) and is expected to have characteristic temperatures in the range of T 105107 K and densities of nH 104106 cm3. Constraining the distribution and physical properties of WHIM is important to understand how the gas (and the metals) are transported from galaxies into the IGM and recycled into galaxies, and what role the shock-heated IGM has for the evolution of galaxies in the local Universe. Since emission from such a thin plasma is extremely dim (Furlanetto et al. 2004; Bertone et al. 2010a,b) and since the hydrogen is almost fully ionized, the analysis of highly ionized heavy elements (in particular the high ions of oxygen, O VI, O VIIand O VIII) in ultraviolet (UV) and X-ray absorption against distant extragalactic background sources has become the leading method to study the properties and baryon content of WHIM at low redshift (for a recent review, see Richter, Paerels & Kaastra 2008). The most promising is the search for five-times ionized oxygen (O VI) in the UV spectra of low-redshift AGN, as obtained with space-based UV spectrographs such as Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) (e.g. Tripp, Savage & Jenkins 2000; Richter et al. 2004). Oxygen is a relatively abundant element with two strong O VI transitions at 1031.9 and 1037.6 , and so far, more than 50 intervening O VI absorbers at low redshift have been identified and analysed (e.g. Tripp et al. 2008; Danforth & Shull 2008). In spite of the large O VI samples obtained to date, there is still no general consensus about the physical conditions of the gas giving rise to O VI absorption. While Danforth & Shull (2008) find that O VI (and associated N V) are reliable tracers of collisionally ionized gas at temperatures 105 K < T < 106 K (i.e. the low-temperature WHIM), Thom & Chen (2008a) argue that O VI arises mainly in photoionized gas at temperatures T < 105 K. Similarly, Tripp et al. (2008) find that well-aligned O VIH I absorbers have linewidths that are consistent with photoionized gas. Nevertheless, these authors show that more than half of their O VI absorbers are complex, i.e. multiphase, and could thus trace both cold photoionized gas and lower-metallicity collisionally ionized gas at T > 105 K. The interpretation of the abundance and nature of intervening O VI absorbers arising in collisionally ionized gas in terms of the distribution and baryon content of WHIM is not straight-forward. In collisional ionization equilibrium (CIE; as usually assumed for a shock-heated plasma like WHIM), O VI predominantly traces gas at T 3 105 K, while in the higher temperature regime of WHIM oxygen is further ionized to O VII and O VIII, observable only in the X-ray band for which (...truncated)