1998A&A...335...33S

We present a detailed analysis of a high resolution spectrum of the damped Lyα system at z_abs=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)∼6x10¹⁶cm^–2 and the fractional abundance of H₂, f=5.4x10^–5. The excitation temperature derived for different transitions suggests that the kinetic temperature of the cloud is ∼200K and the density n∼1000cm^–3. The cloud has therefore a dimension along the line of sight smaller than 1pc. Since it obscures the broad-line emission region, its transverse dimension should be larger than 10pc. We obtain upper limits on the column densities of CI (<10^12.7cm^–2) and CO (<10^13.2cm^–2; N(CO)/N(HI)<7x10^–9). We suggest that the ratio N(H₂)/N(CI) 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(CI)/N(H₂) and N(MgI)/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.