Physical conditions in diffuse interstellar medium of local and high redshift galaxies: measurements based on excitation of H2 rotational and CI fine-structure levels
Abstract
We present results of analysis of physical conditions (number density, intensity of UV field, kinetic temperature) in the cold H2-bearing interstellar medium of local and high redshift galaxies. Our measurements based on the fit to the observed population of H2 rotational levels and CI fine-structure levels with the help of grids of numerical models calculated with the PDR Meudon code. A joint analysis of low H2 rotational levels and CI fine-structure levels allows to break the degeneracy in the IUV-nH plane and provides significantly tighter constraints on the number density and intensity of UV field. Using archive data from the VLT/UVES, KECK/HIRES, HST/STIS and FUSE telescopes we selected 12 high redshift damped Lyα systems (DLAs) in quasar spectra and 14 H2 absorption systems along the lines of sight towards stars in the Milky-Way and the Magellanic Clouds galaxies. These systems have strong H2 components with the column density N(H2)/cm-2>18 and associated CI absorptions. We find that H2-bearing medium in high redshift DLAs and in local galaxies has similar values of the kinetic temperatures Tkin100K and number density 10-500 cm-3. However, the intensity of incident UV radiation in DLAs is varied in the wide range (0.1-100 units of Mathis field), while it is 0.1-3 units of Mathis field for H2 systems in Milky-Way and LMC and SMC galaxies. The large dispersion of measured UV flux in DLAs is probably a consequence that DLA sample probes the galaxies selected from the overall galaxy population at high-z and therefore corresponds to the wide range of the physical conditions.