Astron. Astrophys. 357, 414-428 (2000)

Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255 ^*

R. Srianand ¹ and P. Petitjean <sup>2,3

1</sup> IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India
² Institut d'Astrophysique de Paris - CNRS, 98bis Boulevard Arago, 75014 Paris, France
³ UA CNRS 173 - DAEC, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France

Received 25 October 1999 / Accepted 13 March 2000

Abstract

Results of a careful analysis of the absorption systems with   seen toward the bright,  3.91, gravitationally lensed quasar APM 08279+5255 are presented.

Two of the narrow-line systems, at  = 3.8931 and  = 3.9135, show absorptions from singly ionized species with weak or no N V  and O VI  absorptions at the same redshift. Absorption due to fine structure transitions of C II  and Si II   (excitation energies corresponding to, respectively, 156µm and 34µm) are detected at  = 3.8931. Excitation by IR radiation is favored as the column density ratios are consistent with the shape of APM 08279+5255 IR spectrum. The low-ionization state of the system favors a picture where the cloud is closer to the IR source than to the UV source, supporting the idea that the extension of the IR source is larger than 200 pc. The absence of fine structure lines at  = 3.9135 suggests that the gas responsible for this system is farther away from the IR source. Abundances are 0.01 and 1  at  = 3.913 and 3.8931 and aluminum could be over-abundant with respect to silicon and carbon by at least a factor of two and five. All this suggests that whereas the = 3.8931 system is probably located within 200 pc from the QSO and ejected at a velocity larger than 1000 km s^-1, the = 3.9135 system is farther away and part of the host-galaxy.

Several narrow-line systems have strong absorption lines due to C IV , O VI   and N V  and very low neutral hydrogen optical depths. This probably implies metallicities although firm conclusion cannot be drawn as the exact value depends strongly on the shape of the ionizing spectrum.

The C IV  broad absorption has a complex structure with mini-BALs (width  km s^-1) and narrow components superposed on a continuous absorption of smaller optical depth. The continuous absorption is much stronger in O VI  indicating that the corresponding gas-component is of higher ionization than the other components in the flow and that absorption structures in the BAL-flow are mainly due to density inhomogeneities. There is a tendency for mini-BALs to have different covering factors for different species. It is shown that a few of the absorbing clouds do not cover all the three QSO images, especially we conclude that the  = 3.712 system covers only image C.

Finally we identify narrow components within the BAL-flow with velocity separations within 5 km s^-1 of the O VI , N V  and Si IV  doublet splittings suggesting that line driven radiative acceleration is an important process to explain the out-flow.

Key words: galaxies: quasars: absorption lines galaxies: quasars: individual: APM 08279+5255 galaxies: halos galaxies: ISM

* 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: R. Srianand

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000

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