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Title:
Strong z ~ 0.5 OVI absorption towards PKS 0405-123: implications for ionization and metallicity of the Cosmic Web
Authors:
Howk, J. Christopher; Ribaudo, Joseph S.; Lehner, Nicolas; Prochaska, J. Xavier; Chen, Hsiao-Wen
Affiliation:
AA(Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA), AB(Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA), AC(Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA), AD(UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA), AE(Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 396, Issue 4, pp. 1875-1894. (MNRAS Homepage)
Publication Date:
07/2009
Origin:
MNRAS
MNRAS Keywords:
intergalactic medium , quasars: absorption lines , quasars: individual: PKS 0405-123
DOI:
10.1111/j.1365-2966.2009.14805.x
Bibliographic Code:
2009MNRAS.396.1875H

Abstract

We present observations of the intervening OVI absorption-line system at zabs = 0.495096 towards the quasi-stellar object (QSO) PKS 0405-123 (zem = 0.5726) obtained with the Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. In addition to strong OVI, with , and moderate HI, with , this absorber shows absorption from CIII, NIV, OIV and OV, with upper limits for another seven ions. The large number of available ions allows us to test ionization models usually adopted with far fewer constraints. We find that the observed ionic column densities cannot be matched by single-temperature collisional ionization models, in or out of equilibrium. Photoionization models can match all of the observed column densities, including OVI. If one assumes photoionization by an ultraviolet (UV) background dominated by QSOs, the metallicity of the gas is [O/H] ~ -0.15, while if one assumes a model for the UV background with contributions from ionizing photons escaping from galaxies the metallicity is [O/H] ~ -0.62. Both give [N/O] ~ -0.6 and [C/H] ~ -0.2 to ~-0.1, though a solar C/O ratio is not ruled out. The choice of ionizing spectrum is poorly constrained and leads to systematic abundance uncertainties of ~0.5 dex, despite the wide range of available ions. Multiphase models with a contribution from both photoionized gas (at T ~ 104 K) and collisionally ionized gas [at T ~ (1-3) × 105 K] can also match the observations for either assumed UV background giving very similar metallicities. We do not detect NeVIII or MgX absorption. The limit on NeVIII/OVI < 0.21 (3σ) is the lowest yet observed. Thus, this absorber shows no firm evidence of the `warm-hot intergalactic medium' at T ~ (0.5-3) × 106K thought to contain a significant fraction of the baryons at low redshift. The OVI in this system is not necessarily a reliable tracer of the warm-hot intergalactic medium given the ambiguity in its origins. We present limits on the total column of warm-hot gas in this absorber as a function of temperature. This system would be unlikely to provide detectable X-ray absorption in the ions OVII or OVIII even if it resided in front of the brighter X-ray sources in the sky.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985. Also based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 7576.

E-mail: jhowk@nd.edu

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