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Astron. Astrophys. 335, 33-40 (1998)

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Molecules in the zabs = 2.8112 damped system toward PKS 0528-250 *

R. Srianand 1 and Patrick Petitjean 2, 3

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

Received 22 August 1997 / Accepted 16 March 1998

Abstract

We present a detailed analysis of a high resolution spectrum of the damped Ly [FORMULA] system at [FORMULA]  = 2.8112 toward PKS 0528-250. The absorption redshift is slighly larger than the emission redshift of the quasar. We estimate the column density of H2 molecules N (H2[FORMULA]  6 [FORMULA] 1016  cm-2 and the fractional abundance of H2, f  = 5.4 [FORMULA] 10-5. The excitation temperature derived for different transitions suggests that the kinetic temperature of the cloud is [FORMULA] 200 K and the density n   [FORMULA]  1000 cm-3. The cloud has therefore a dimension along the line of sight smaller than 1 pc. Since it obscures the broad-line emission region, its transverse dimension should be larger than 10 pc.

We obtain upper limits on the column densities of C I ([FORMULA] [FORMULA] cm-2) and CO ([FORMULA]   [FORMULA]  cm-2 ; N (CO)/N (H I) [FORMULA] 7 10-9). We suggest that the ratio N (H2)/N (C I) is a useful indicator of the physical conditions in the absorber. Simple photo-ionization models assuming solar relative abundances show that radiation fields with spectra similar to typical AGNs or starbursts are unable to reproduce all the constraints and in particular the surprizingly small N (C I)/N (H2) and N (Mg I)/N (H2) ratios. In view of the models we explored, the most likely ionizing spectrum is a composite of a UV-"big bump" possibly produced by a local starburst and a power-law spectrum from the QSO that provides the X-rays. Dust is needed to explain the production of molecules in the cloud. The amount of dust is broadly consistent with the [Cr/Zn] abundance determination.

Key words: galaxies: ISM – quasars: absorption lines – quasars: individual: PKS 0528-250 – galaxies: halos

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: R. Srianand

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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