Physical conditions in diffuse interstellar medium of local and high redshift galaxies: measurements based on excitation of H${}_2$ 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 H${}_2$-bearing interstellar medium of local and high redshift galaxies. Our measurements 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 PDR Meudon code. A joint analysis of low H${}_2$ rotational levels and CI fine-structure levels allows to break the degeneracy in the $I_{UV}-n_{\mathrm{H}}$ 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 Ly$\alpha$ 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 Clouds galaxies. These systems have strong H${}_2$ components with the column density $\log N({\mathrm{H}}_2)/{\mathrm{cm}}^{-2}>18$ and associated CI absorptions. We find that H${}_2$-bearing medium in high redshift DLAs and in local galaxies has similar values of the kinetic temperatures $T_{\mathrm{kin}}\sim 100$K 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 $\sim 0.1-3$ units of Mathis field for H${}_2$ 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.

20 pages, 14 figures, accepted for publication in MNRAS

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• A detection of H${}_2$ in a high velocity cloud toward the Large Magellanic Cloud
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• Extremely strong DLAs at high redshift: Gas cooling and H${}_2$ formation
• Molecular hydrogen in absorption at high redshifts. Science cases for CUBES