Molecules in the z abs = 2.8112 damped system toward PKS 0528--250

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We present a detailed analysis of a high resolution spectrum of the damped Lyα system at z abs = 2.8112toward PKS 0528-250. The absorption redshift is slightly larger than the emission redshift of the quasar. We estimate the column density of H₂ molecules N(H₂) 6×10¹⁶ cm⁻² and the fractional abundance of H₂, f = 5.4×10⁻⁵. The excitation temperature derived for different transitions suggests that the kinetic temperature of the cloud is 200 K and the density n 1000 cm⁻³. The cloud therefore has a dimension of 1 pc along the line of sight. 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 (< 10¹².⁷ cm⁻²) and CO (< 10¹³.² cm⁻²; N(CO)/N(H i) < 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 surprisingly 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.