Highly-Ionized Oxygen Absorbers in the Intergalactic Medium

Abstract

Recent ultraviolet and X-ray observations of intergalactic OVI and OVII absorption systems along lines of sight to bright quasars have opened a new window onto the ``warm-hot intergalactic medium'' (WHIM). These systems appear to provide a significant reservoir for baryons in the local universe, and comparison to cosmological simulations suggests that their abundance roughly matches theoretical predictions. Here we use analytic arguments to elucidate the physical properties of the absorbers and their role in structure formation. We first show that if the absorbers result from structure-formation shocks, the observed column densities naturally follow from postshock-cooling models, if we include fast-cooling shocks as well as those that cannot cool within a Hubble time. In this case, the known OVI absorbers should show stronger OVII absorption than expected from collisional-ionization equilibrium (and much more than expected for photoionized systems). We then argue that higher-temperature shocks will be spatially associated with more massive virialized objects even well outside the virial radius. Thus the different oxygen ions will trace different structures; OVII absorbers are the most common because that ion dominates over a wide temperature range (corresponding to a large range in halo mass). If each dark-matter halo is surrounded by a network of shocks with total cross section a few times the size of the virialized systems, then we can reproduce the observed number densities of absorbers with plausible parameters. A simple comparison with simulations shows that these assumptions are reasonable, although the actual distribution of shocked gas is too complex for analytic models to describe fully. Our models suggest that these absorbers cannot be explained as a single-temperature phase.

13 pages, 12 figures, in press at MNRAS (minor modifications, conclusions unchanged)

References in corpus (11)

• First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters
• Virial shocks in galactic haloes?
• CAIRNS: The Cluster And Infall Region Nearby Survey I. Redshifts and Mass Profiles
• Chandra Detection of OVIII Ly-alpha Absorption from an Overdense Region in the Intergalactic Medium
• FUSE and STIS Observations of the Warm-Hot Intergalactic Medium towards PG1259+593
• Chandra Detection of Local OVII He-alpha Absorption Along the Sight Line Toward 3C 273
• Probing the IGM/Galaxy Connection Toward PKS0405-123 I: UV Spectroscopy and Metal-Line Systems
• Large-Scale Structure Shocks at Low and High Redshifts
• The Hot Intergalactic Medium - Galaxy Connection: Two Strong Absorbers in the Sightline Toward PG 1211+143
• The Multiphase Intergalactic Medium towards PKS 2155-304
• Can radiative cooling and nongravitational heating explain simultaneously the global X-ray properties of clusters and the unresolved cosmic X-ray background ?

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• The Nature of the Warm/Hot Intergalactic Medium I. Numerical Methods, Convergence, and OVI Absorption
• The Relationship Between Intergalactic HI/OVI and Nearby (z<0.017) Galaxies
• Where Are the Baryons? III: Non-Equilibrium Effects and Observables
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• Confirming the Detection of an Intergalactic X-ray Absorber Toward PKS 2155-304
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• Numerical simulations of the Warm-Hot Intergalactic Medium
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• X-raying the Intergalactic OVI Absorbers
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