Cold diffuse interstellar medium of Magellanic Clouds: II. Physical conditions from excitation of CI and H$_2$

TL;DR

This study investigates the physical conditions of the diffuse interstellar medium in the Magellanic Clouds by analyzing CI and H$_2$ excitation, revealing stronger UV fields and higher pressures than in the local ISM, with implications for high-redshift galaxy environments.

Contribution

It provides the first detailed measurements of CI excitation in the Magellanic Clouds and compares these with PDR models to infer UV fields and pressures, highlighting their relevance to high-redshift galaxy conditions.

Findings

CI excitation levels are 2-3 times higher than in local diffuse ISM.

The UV radiation field in the Magellanic Clouds is stronger than in the Milky Way.

The thermal pressure in the Magellanic Clouds is higher, indicating denser, warmer gas near star-forming regions.

Abstract

We present a comprehensive study of the excitation of CI fine-structure levels along 57 sight lines in the Large and Small Magellanic Clouds. The sight lines were selected by the detection of H$_2$ in FUSE spectra. Using archival HST/COS and HST/STIS spectra we detected absorption of CI fine-structure levels and measured their populations for 29 and 22 sight lines in the LMC and SMC, respectively. The CI column density ranges from $10^{13}$ to $10^{14}\,{\rm cm}^{-2}$ for the LMC and $10^{13}$ to $10^{15.4}\,{\rm cm}^{-2}$ for the SMC. We found excitation of CI fine-structure levels in the LMC and SMC to be 2-3 times higher than typical values in local diffuse ISM. Comparing excitation of both CI fine-structure levels and H$_2$ rotational levels with a grid of PDR Meudon models we find that neutral cold gas in the LMC and SMC is illuminated by stronger UV field than in local ISM ($\chi=5^{+7}_{-3}$ units of Mathis field for the LMC and $2^{+4}_{-1}$ for the SMC) and has on average higher thermal pressure ($\log p/k =4.2\pm0.4$ and $4.3\pm0.5$, respectively). Magellanic Clouds sight lines likely probe region near star-formation sites, which also affects the thermal state and CI/H$_2$ relative abundances. At the same time such high measurements of UV field are consistent with some values obtained at high redshifts. Together with low metallicities this make Magellanic Clouds to be an interesting test case to study of the central parts of high redshift galaxies.