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Astron. Astrophys. 359, 457-470 (2000)

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Structure of the Mg II and damped Lyman-[FORMULA] systems along the line of sight to APM 08279+5255 *

P. Petitjean 1,2, B. Aracil 1, R. Srianand 3 and R. Ibata 4

1 Institut d'Astrophysique de Paris - CNRS, 98bis Boulevard Arago, 75014 Paris, France
2 UA CNRS 173 - DAEC, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
3 IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India
4 Max-Plank-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received 27 January 2000 / Accepted 9 May 2000


A study of the absorption systems toward the gravitationally lensed quasar APM 08279+5255 is presented.

Most of the Mg II systems in the redshift range z [FORMULA] 1.2-2.07, although saturated, show large residuals at the bottom of the lines. The most likely interpretation is that individual clouds within Mg II halos do cover only one of the two brightest QSO images. The separation between the two lines of sight decreases from 1.7 to 0.7 [FORMULA] kpc ([FORMULA] = 0.5, [FORMULA] = 1) between z = 1.22 and z = 2.07. This reveals that Mg II halos are made of a collection of clouds of radius smaller than about 1 [FORMULA] kpc.

Two strong Mg II absorbers at [FORMULA] = 1.062 and 1.181 are studied in detail. This is the first time that the Na I [FORMULA]3303 doublet is detected in such high redshift systems. Together with the detection of the Mg I [FORMULA]2852 transition, this strongly constrains the physical characteristics of the gas. The N(Na I )/N(Mg I ) ratio is found to be larger than unity, implying that the gas is cool and neutral. The Doppler parameters measured in individual and well detached components are probably as small as 1 km s-1. The column densities of Na I , Ca II , Mg I , Ti II , Mn II and Fe II observed at [FORMULA] = 1.1801 are very close to that observed along the line of sight towards 23 Ori in our Galaxy. The shape of the QSO continuum is consistent with attenuation by dust at z [FORMULA] 1 ([FORMULA] [FORMULA] 0.5 mag). Altogether it is found that the H I column density at z = 1 is of the order of 1 to 5 1021 cm-2, the corresponding metallicity is in the range 1-0.3 [FORMULA], the overall dust-to-metal ratio is about half that in our Galaxy and the relative depletion of iron, titanium, manganese and calcium is similar to what is observed in cool gas in the disk of our Galaxy. The objects associated with these two systems could both contribute to the lens together with another possible strong system at [FORMULA] = 1.1727 and the strong Lyman-[FORMULA] system at [FORMULA] = 2.974.

The probable damped Lyman-[FORMULA] system at [FORMULA] = 2.974 has 19.8 [FORMULA] log N(H I[FORMULA] 20.3. The transverse dimension of the absorber is larger than 200 [FORMULA] pc. Column densities of Al II , Fe II , Si II , C II and O I indicate abundances relative to solar of -2.31, -2.26, -2.10, -2.35 and -2.37 for, respectively, Fe, Al, Si, C and O (for log N(H I ) = 20.3). These surprizingly similar values indicate that the amount of dust in the cloud is very small as are any deviations from relative solar abundances. It seems likely that the upper limits found for the zinc metallicity of several damped Lyman-[FORMULA] systems at z [FORMULA] 3 in previous surveys is indicative of a true cosmological evolution of the metallicity in individual systems.

Key words: galaxies: quasars: absorption lines – galaxies: quasars: individual: APM08279+5255

* Based on observations collected at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

Send offprint requests to: P. Petitjean

© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000