Clues to the nature of high-redshift O vi absorption systems from their lack of small-scale structure ^*,**

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

Corresponding author: tkim@aip.de

Received: 28 March 2006
Accepted: 13 March 2007

Abstract

We present results of the first survey of high-redshift  absorption systems along parallel lines of sight toward two lensed QSOs. After a careful and well-defined search, we find ten intervening  systems – identified by the presence of the 1031, 1037 doublet lines, , and in most cases , , and  – and eight candidate systems for which we do not detect  nor other metals. We assess the veracity of these systems by applying a classification scheme. Within the errors, all systems appear at the same redshift and have similar line strengths in front of both QSO images, whereas in most cases  or  show more differences across the lines of sight, either in radial velocity or line strength. We conclude that (1) the coherence length of  must be much larger than 1  kpc, and (2) an important fraction of the  absorbers may not reside in the same volume as . Given the inhomogeneous character of the data – different 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  occurs in two different photoionized phases, one large, with characteristic sizes of a few hundred kpc and bearing , and another one a factor of ten smaller and containing . 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] ≈ -0.7 is required to explain the observations, which hints at late () rather than early metal enrichment. In the high-metallicity regime, the observed dissociation between  and  gas might be produced by galactic outflows. Altogether, the relative abundances, inhomogeneous   and featureless  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 that also show .