Physical conditions in the diffuse interstellar medium of local and high-redshift galaxies: measurements based on the excitation of H2 rotational and CI fine-structure levels

We present the results of an analysis of the physical conditions (number density, intensity of UV field, kinetic temperature) in the cold H-2-bearing interstellar medium of local and high-redshift galaxies. Our measurements are based on the fit to the observed population of H-2 rotational levels and CI fine-structure levels with the help of grids of numerical models calculated with the photon-dominated region (PDR) MEUDON code. A joint analysis of low H-2 rotational levels and CI fine-structure levels breaks the degeneracy in the I-UV-n(H) plane and provides significantly tighter constraints on the number density and intensity of the UV field. Using archive data from the VLT/UVES, KECK/HIRES, HST/STIS and FUSE telescopes, we selected 12 high-redshift damped Lya systems (DLAs) in quasar spectra and 14 H-2 absorption systems along the lines of sight towards stars in the Milky Way and the Magellanic Cloud galaxies. These systems have strong H-2 components, with a column density logN(H-2)/[cm(-2)] > 18 and associated C I absorptions. We find that H-2-bearing media in high-redshift DLAs and in local galaxies have similar values of the kinetic temperatures T-kin similar to 100K and number density 10-500 cm(-3). However, the intensity of incident UV radiation in DLAs varies in a wide range (0.1-100 units of the Mathis field), while it is similar to 0.1-3 units of the Mathis field for H-2 systems in the Milky Way and Large and Small Magellanic Cloud galaxies. The large dispersion of measured UV flux in DLAs is probably a consequence of the fact that the DLA sample probes galaxies selected from the overall galaxy population at high redshift, and therefore corresponds to a wide range of physical conditions.