Astronomy & Astrophysics

Table of contents

A&A 469, 61-74 (2007)
DOI: 10.1051/0004-6361:20065301

Clues to the nature of high-redshift O VI absorption systems from their lack of small-scale structure

S. Lopez¹, S. Ellison², S. D'Odorico³, and T.-S. Kim⁴

¹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
    e-mail: slopez@das.uchile.cl
² University of Victoria, Dept. Physics & Astronomy, Elliott Building, 3800 Finnerty Rd, Victoria, V8P 1A1, British Columbia, Canada
    e-mail: sarae@uvic.ca
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching-bei-München, Germany
    e-mail: sdodoric@eso.org
⁴ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

(Received 28 March 2006 / Accepted 13 March 2007)

Abstract
We present results of the first survey of high-redshift $(\langle z\rangle\sim 2.3)$ $\ion{O}{vi}$ absorption systems along parallel lines of sight toward two lensed QSOs. After a careful and well-defined search, we find ten intervening $\ion{O}{vi}$ systems - identified by the presence of the $\lambda\lambda$ 1031, 1037 doublet lines, $\ion$ , and in most cases $\ion{C}{iv}$ , $\ion{iv}$ , and $\ion{C}{iii}$ - and eight candidate systems for which we do not detect $\ion$ nor other metals. We assess the veracity of these systems by applying a classification scheme. Within the errors, all $\ion{O}{vi}$ systems appear at the same redshift and have similar line strengths in front of both QSO images, whereas in most cases $\ion{C}{iv}$ or $\ion{iv}$ show more differences across the lines of sight, either in radial velocity or line strength. We conclude that (1) the coherence length of $\ion{O}{vi}$ must be much larger than $\approx$ 1 h₇₀^-1 kpc, and (2) an important fraction of the $\ion{C}{iv}$ absorbers may not reside in the same volume as $\ion{O}{vi}$ . Given the inhomogeneous character of the data - different S/N ratios and degrees of blending - we pay special attention to the observational errors and their impact on the above conclusions. Since Doppler parameters are consistent with photoionization, we propose a model in which $\ion{C}{iv}$ occurs in two different photoionized phases, one large, with characteristic sizes of a few hundred kpc and bearing $\ion{O}{vi}$ , and another one a factor of ten smaller and containing $\ion{C}{iii}$ . This model is able to explain the various transverse differences observed in column density and kinematics. We apply the model successfully to 2 kinds of absorbers, with low and high metallicity. In the low-metallicity regime, [C/H]~ -2, we find that [C/O] $\approx$ -0.7 is required to explain the observations, which hints at late ( $z\la 6$ ) rather than early metal enrichment. In the high-metallicity regime, the observed dissociation between $\ion{O}{vi}$ and $\ion{C}{iv}$ gas might be produced by galactic outflows. Altogether, the relative abundances, inhomogeneous $\ion{C}{iv}$ and featureless $\ion{O}{vi}$ are consistent with gas that has been processed recently before the absorption occurred (thus close to star-forming regions). Finally, we discuss briefly three associated systems $(z_{\rm abs} \sim z_{\rm em})$ that also show $\ion{O}{vi}$ .

Key words: cosmology: observations -- intergalactic medium -- galaxies: halos -- galaxies: quasars: absorption lines -- quasars: individual: HE1104-1805 -- quasars individual: RX J0911.4+0551