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

TL;DR

This study analyzes the physical conditions of the cold interstellar medium in local and high-redshift galaxies using H$_2$ rotational and CI fine-structure level excitation, revealing similarities and differences in UV radiation and density.

Contribution

It introduces a joint analysis method combining H$_2$ and CI data to better constrain density and UV field strength in diverse galactic environments.

Findings

High-redshift DLAs and local galaxies have similar kinetic temperatures (~100K) and densities (10-500 cm$^{-3}$).

UV radiation intensity varies widely in DLAs, indicating diverse physical conditions.

The joint analysis reduces degeneracy in determining interstellar medium parameters.

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 $I_{UV}-n_{\rm{H}}$ 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 $\log N({\rm{H_2}})/{\rm{cm}}^{-2}>18$ 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 $T_{\rm{kin}}\sim100$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 $\sim0.1-3$ 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.