Physical conditions in quasi-stellar object absorbers from fine-structure absorption lines

We calculate theoretical population ratios of the ground fine-structure levels of some atoms/ions which typically exhibit ultraviolet (UV) lines in the spectra of quasi-stellar objects (QSO) absorbers redward the Lyα forest: C⁰, C⁺, O⁰, Si⁺ and Fe⁺. The most reliable atomic data available are employed and a variety of excitation mechanisms are considered: collisions with several particles in the medium, direct excitation by photons from the cosmic microwave background radiation (CMBR) and fluorescence induced by a UV field present. The theoretical population ratios are compared with the corresponding column density ratios of CI and CII lines observed in damped Lyα (DLA) and Lyman Limit (LL) systems, collected in the recent literature, to infer their physical conditions. The volumetric density of neutral hydrogen in DLA systems is constrained to be lower than tens of cm^-3 (or a few cm^-3 in the best cases), and upper limits to the UV radiation field intensities to be about two orders of magnitude bigger than the radiation field of the Galaxy (one order of magnitude in the best cases). Their characteristic sizes are higher than a few pc (tens of pc in the best cases) and lower limits for their total masses vary from 10⁰ to 10⁵ solar masses. For the only LL system in our sample, the electronic density is constrained to be n_e<0.15cm^-3. We suggest that the fine-structure lines may be used to discriminate between the current accepted picture of the UV extragalactic background as the source of ionization in these systems and a local origin for the ionizing radiation as supported by some authors. We also investigate the validity of the temperature-redshift relation of the CMBR predicted by the standard model and study the case for alternative models.